Co Nanoparticles Confined in 3D Nitrogen‐Doped Porous Carbon Foams as Bifunctional Electrocatalysts for Long‐Life Rechargeable Zn–Air Batteries

Proper design and simple preparation of nonnoble bifunctional electrocatalysts with high cost performance and strong durability for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is highly demanded but still full of enormous challenges. In this work, a spontaneous gas‐foaming strategy is presented to synthesize cobalt nanoparticles confined in 3D nitrogen‐doped porous carbon foams (CoNCF) by simply carbonizing the mixture of citric acid, NH₄Cl, and Co(NO₃)₂·6H₂O. Thanks to its particular 3D porous foam architecture, ultrahigh specific surface area (1641 m² g^?1), and homogeneous distribution of active sites (C–N, Co–N_x, and Co–O moieties), the optimized CoNCF‐1000‐80 (carbonized at 1000 °C, containing 80 mg Co(NO₃)₂·6H₂O in precursors) catalyst exhibits a remarkable bifunctional activity and long‐term durability toward both ORR and OER. Its bifunctional activity parameter (ΔE) is as low as 0.84 V, which is much smaller than that of noble metal catalyst and comparable to state‐of‐the‐art bifunctional catalysts. When worked as an air electrode catalyst in rechargeable Zn–air batteries, a high energy density (797 Wh kg^?1), a low charge/discharge voltage gap (0.75 V), and a long‐term cycle stability (over 166 h) are achieved at 10 mA cm^?2.     

Strongly Coupled Carbon Nanosheets/Molybdenum Carbide Nanocluster Hollow Nanospheres for High‐Performance Aprotic Li–O2 Battery

A highly efficient oxygen electrode is indispensable for achieving high‐performance aprotic lithium–O₂ batteries. Herein, it is demonstrated that strongly coupled carbon nanosheets/molybdenum carbide (α‐MoC_1?x) nanocluster hierarchical hybrid hollow spheres (denoted as MoC_1?x/HSC) can work well as cathode for boosting the performance of lithium–O₂ batteries. The important feature of MoC_1?x/HSC is that the α‐MoC_1?x nanoclusters, uniformly incorporated into carbon nanosheets, can not only effectively prevent the nanoclusters from agglomeration, but also help enhance the interaction between the nanoclusters and the conductive substrate during the charge and discharge process. As a consequence, the MoC_1?x/HSC shows significantly improved electrocatalytic performance in an aprotic Li–O₂ battery with greatly reduced charge and discharge overpotentials and long cycle stability. The ex situ scanning electron microscopy, X‐ray diffraction, and X‐ray photoelectron spectroscopy studies reveal that the mechanism for the high‐performance Li–O₂ battery using MoC_1?x/HSC as cathode is that the incorporated molybdenum carbide nanoclusters can make oxygen reduction on their surfaces easy, and finally form amorphous film‐like Li‐deficient Li₂O₂ with the ability to decompose at a low potential. To the best of knowledge, the MoC_1?x/HSC of this paper is among the best cathode materials for lithium–O₂ batteries reported to date.     

Yolk–Shelled C@Fe3O4 Nanoboxes as Efficient Sulfur Hosts for High‐Performance Lithium–Sulfur Batteries

Owing to the high theoretical specific capacity (1675 mA h g^?1) and low cost, lithium–sulfur (Li–S) batteries offer advantages for next‐generation energy storage. However, the polysulfide dissolution and low electronic conductivity of sulfur cathodes limit the practical application of Li–S batteries. To address such issues, well‐designed yolk–shelled carbon@Fe₃O₄ (YSC@Fe₃O₄) nanoboxes as highly efficient sulfur hosts for Li–S batteries are reported here. With both physical entrapment by carbon shells and strong chemical interaction with Fe₃O₄ cores, this unique architecture immobilizes the active material and inhibits diffusion of the polysulfide intermediates. Moreover, due to their high conductivity, the carbon shells and the polar Fe₃O₄ cores facilitate fast electron/ion transport and promote continuous reactivation of the active material during the charge/discharge process, resulting in improved electrochemical utilization and reversibility. With these merits, the S/YSC@Fe₃O₄ cathodes support high sulfur content (80 wt%) and loading (5.5 mg cm^?2) and deliver high specific capacity, excellent rate capacity, and long cycling stability. This work provides a new perspective to design a carbon/metal‐oxide‐based yolk–shelled framework as a high sulfur‐loading host for advanced Li–S batteries with superior electrochemical properties.     

Plasma Treatment for Nitrogen‐Doped 3D Graphene Framework by a Conductive Matrix with Sulfur for High‐Performance Li–S Batteries

Carbon materials have received considerable attention as host cathode materials for sulfur in lithium–sulfur batteries; N‐doped carbon materials show particularly high electrocatalytic activity. Efforts are made to synthesize N‐doped carbon materials by introducing nitrogen‐rich sources followed by sintering or hydrothermal processes. In the present work, an in situ hollow cathode discharge plasma treatment method is used to prepare 3D porous frameworks based on N‐doped graphene as a potential conductive matrix material. The resulting N‐doped graphene is used to prepare a 3D porous framework with a S content of 90 wt% as a cathode in lithium–sulfur cells, which delivers a specific discharge capacity of 1186 mAh g^?1 at 0.1 C, a coulombic efficiency of 96% after 200 cycles, and a capacity retention of 578 mAh g^?1 at 1.0 C after 1000 cycles. The performance is attributed to the flexible 3D structure and clustering of pyridinic N‐dopants in graphene. The N‐doped graphene shows high electrochemical performance and the flexible 3D porous stable structure accommodates the considerable volume change of the active material during lithium insertion and extraction processes, improving the long‐term electrochemical performance.     

TiO2 and Co Nanoparticle‐Decorated Carbon Polyhedra as Efficient Sulfur Host for High‐Performance Lithium–Sulfur Batteries

Metal organic frameworks (MOFs)‐derived porous carbon is proposed as a promising candidate to develop novel, tailorable structures as polysulfides immobilizers for lithium–sulfur batteries because of their high‐efficiency electron conductive networks, open ion channels, and abundant central ions that can store a large amount of sulfur and trap the easily soluble polysulfides. However, most central ions in MOFs‐derived carbon framework are encapsulated in the carbon matrix so that their exposures as active sites to adsorb polysulfides are limited. To resolve this issue, highly dispersed TiO₂ nanoparticles are anchored into the cobalt‐containing carbon polyhedras that are converted from ZIF‐67. Such a type of TiO₂ and Co nanoparticles‐decorated carbon polyhedras (C? Co/TiO₂) provide more exposed active sites and much stronger chemical trapping for polysulfides, hence improving the sulfur utilization and enhancing reaction kinetics of sulfur‐containing cathode simultaneously. The sulfur‐containing carbon polyhedras decorated with TiO₂ nanoparticles (S@C? Co/TiO₂) show a significantly improved cycling stability and rate capability, and deliver a discharge capacity of 32.9% higher than that of TiO₂‐free S@C? Co cathode at 837.5 mA g^?1 after 200 cycles.     

Strategies toward High‐Performance Cathode Materials for Lithium–Oxygen Batteries

Rechargeable aprotic lithium (Li)–O₂ batteries with high theoretical energy densities are regarded as promising next‐generation energy storage devices and have attracted considerable interest recently. However, these batteries still suffer from many critical issues, such as low capacity, poor cycle life, and low round‐trip efficiency, rendering the practical application of these batteries rather sluggish. Cathode catalysts with high oxygen reduction reaction (ORR) and evolution reaction activities are of particular importance for addressing these issues and consequently promoting the application of Li–O₂ batteries. Thus, the rational design and preparation of the catalysts with high ORR activity, good electronic conductivity, and decent chemical/electrochemical stability are still challenging. In this Review, the strategies are outlined including the rational selection of catalytic species, the introduction of a 3D porous structure, the formation of functional composites, and the heteroatom doping which succeeded in the design of high‐performance cathode catalysts for stable Li–O₂ batteries. Perspectives on enhancing the overall electrochemical performance of Li–O₂ batteries based on the optimization of the properties and reliability of each part of the battery are also made. This Review sheds some new light on the design of highly active cathode catalysts and the development of high‐performance lithium–O₂ batteries.     

Anode Interface Engineering and Architecture Design for High‐Performance Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries are considered as one of the most promising options to realize rechargeable batteries with high energy capacity. Previously, research has mainly focused on solving the polysulfides' shuttle, cathode volume changes, and sulfur conductivity problems. However, the instability of anodes in Li–S batteries has become a bottleneck to achieving high performance. Herein, the main efforts to develop highly stable anodes for Li–S batteries, mainly including lithium metal anodes, carbon‐based anodes, and alloy‐based anodes, are considered. Based on these anodes, their interfacial engineering and structure design are identified as the two most important directions to achieve ideal anodes. Because of high reactivity and large volume change during cycling, Li anodes suffer from severe side reactions and structure collapse. The solid electrolyte interphase formed in situ by modified electrolytes and ex situ artificial coating layers can enhance the interfacial stability of anodes. Replacing common Li foil with rationally designed anodes not only suppresses the formation of dendritic Li but also delays the failure of Li anodes. Manipulating the anode interface engineering and rationally designing anode architecture represents an attractive path to develop high‐performance Li–S batteries.     

Superhierarchical Cobalt‐Embedded Nitrogen‐Doped Porous Carbon Nanosheets as Two‐in‐One Hosts for High‐Performance Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries, based on the redox reaction between elemental sulfur and lithium metal, have attracted great interest because of their inherently high theoretical energy density. However, the severe polysulfide shuttle effect and sluggish reaction kinetics in sulfur cathodes, as well as dendrite growth in lithium‐metal anodes are great obstacles for their practical application. Herein, a two‐in‐one approach with superhierarchical cobalt‐embedded nitrogen‐doped porous carbon nanosheets (Co/N‐PCNSs) as stable hosts for both elemental sulfur and metallic lithium to improve their performance simultaneously is reported. Experimental and theoretical results reveal that stable Co nanoparticles, elaborately encapsulated by N‐doped graphitic carbon, can work synergistically with N heteroatoms to reserve the soluble polysulfides and promote the redox reaction kinetics of sulfur cathodes. Moreover, the high‐surface‐area pore structure and the Co‐enhanced lithiophilic N heteroatoms in Co/N‐PCNSs can regulate metallic lithium plating and successfully suppress lithium dendrite growth in the anodes. As a result, a full lithium–sulfur cell constructed with Co/N‐PCNSs as two‐in‐one hosts demonstrates excellent capacity retention with stable Coulombic efficiency.     

High‐Capacity and High‐Rate Discharging of a Coenzyme Q10‐Catalyzed Li–O2 Battery

Discharging of the aprotic Li–O₂ battery relies on O₂ reduction to insulating solid Li₂O₂, which can either deposit as thin films on the cathode surface or precipitate as large particles in the electrolyte solution. Toward realizing Li–O₂ batteries with high capacity and high rate capability, it is crucially important to discharge Li₂O₂ in the electrolyte solution rather than on the cathode surface. Here, a soluble electrocatalyst of coenzyme Q₁₀ (CoQ₁₀) that can efficaciously drive solution phase formation of Li₂O₂ in current benchmark ether‐based Li–O₂ batteries is reported, which would otherwise lead to Li₂O₂ surface‐film growth and premature cell death. In the range of current densities of 0.1–0.5 mA cm^?2_areal, the CoQ₁₀‐catalyzed Li–O₂ battery can deliver a discharge capacity that is ≈40–100 times what the pristine Li–O₂ battery could achieve. The drastically enhanced electrochemical performance is attributed to the CoQ₁₀ that not only efficiently mediates the electron transfer from the cathode to dissolve O₂ but also strongly interacts with the newly formed Li₂O₂ in solution retarding its precipitation on the cathode surface. The mediated oxygen reduction reaction and the bonding mechanism between CoQ₁₀ and Li₂O₂ are understood with density functional theory calculations.     

Redox Mediators for Li–O2 Batteries: Status and Perspectives

Li–O₂ batteries have received much attention due to their extremely large theoretical energy density. However, the high overpotentials required for charging Li–O₂ batteries lower their energy efficiency and degrade the electrolytes and carbon electrodes. This problem is one of the main obstacles in developing practical Li–O₂ batteries. To solve this problem, it is important to facilitate the oxidation of Li₂O₂ upon charging by using effective electrocatalysis. Using solid catalysts is not too effective for oxidizing the electronically isolating Li‐peroxide layers. In turn, for soluble catalysts, red‐ox mediators (RMs) are homogeneously dissolved in the electrolyte solutions and can effectively oxidize all of the Li₂O₂ precipitated during discharge. RMs can decompose solid Li₂O₂ species no matter their size, morphology, or thickness and thus dramatically increase energy efficiency. However, some negative side effects, such as the shuttle reactions of RMs and deterioration of the Li‐metal occur. Therefore, it is necessary to study the activity and stability of RMs in Li–O₂ batteries in detail. Herein, recent studies related to redox mediators are reviewed and the mechanisms of redox reactions are illustrated. The development opportunities of RMs for this important battery technology are discussed and future directions are suggested.     

Protecting the Li‐Metal Anode in a Li–O2 Battery by using Boric Acid as an SEI‐Forming Additive

The Li–O₂ battery (LOB) is considered as a promising next‐generation energy storage device because of its high theoretic specific energy. To make a practical rechargeable LOB, it is necessary to ensure the stability of the Li anode in an oxygen atmosphere, which is extremely challenging. In this work, an effective Li‐anode protection strategy is reported by using boric acid (BA) as a solid electrolyte interface (SEI) forming additive. With the assistance of BA, a continuous and compact SEI film is formed on the Li‐metal surface in an oxygen atmosphere, which can significantly reduce unwanted side reactions and suppress the growth of Li dendrites. Such an SEI film mainly consists of nanocrystalline lithium borates connected with amorphous borates, carbonates, fluorides, and some organic compounds. It is ionically conductive and mechanically stronger than conventional SEI layer in common Li‐metal‐based batteries. With these benefits, the cycle life of LOB is elongated more than sixfold.     

Oxygen Reduction Reaction Promotes Li+ Desorption from Cathode Surface in Li‐O2 Batteries

Li‐O₂ batteries are claimed to be one of the future energy storage technologies. Great number of scientific and technological challenges should be solved first to transform Li‐O₂ battery from a promise to real practical devices. Proposed mechanisms for oxygen reduction assume a reservoir of solved Li⁺ ions in the electrolyte. However, the role that adsorbed Li⁺ on the electrode surface might have on the overall oxygen reduction reaction (ORR) has not deserved much attention. Adsorbed Li⁺ consumption is monitored here using impedance measurements from extended electrochemical double layer capacitance, which depends on the carbon matrix surface area. The presence of O₂ drastically reduces the amount of adsorbed Li⁺, signaling the kinetic competition between Li⁺ surface adsorption and its consumption, only for potentials corresponding to the oxygen reduction reaction. Noticeably double layer capacitance remains unaltered after cycling. This fact suggests that the ORR products (Li₂O₂ and Li₂CO₃) are not covering the internal electrode surface, but deposited on the outer electrode‐contact interface, hindering thereby the subsequent reaction. Current results show new insights into the discharge mechanism of Li‐O₂ batteries and reveal the evidence of Li⁺ desorption from the C surface when the ORR starts.