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.     

Lithium–Sulfur Batteries: Progress and Prospects

Development of advanced energy‐storage systems for portable devices, electric vehicles, and grid storage must fulfill several requirements: low‐cost, long life, acceptable safety, high energy, high power, and environmental benignity. With these requirements, lithium–sulfur (Li–S) batteries promise great potential to be the next‐generation high‐energy system. However, the practicality of Li–S technology is hindered by technical obstacles, such as short shelf and cycle life and low sulfur content/loading, arising from the shuttling of polysulfide intermediates between the cathode and anode and the poor electronic conductivity of S and the discharge product Li₂S. Much progress has been made during the past five years to circumvent these problems by employing sulfur–carbon or sulfur–polymer composite cathodes, novel cell configurations, and lithium‐metal anode stabilization. This Progress Report highlights recent developments with special attention toward innovation in sulfur‐encapsulation techniques, development of novel materials, and cell‐component design. The scientific understanding and engineering concerns are discussed at the end in every developmental stage. The critical research directions needed and the remaining challenges to be addressed are summarized in the Conclusion.     

Intrinsically Optimizing Charge Transfer via Tuning Charge/Discharge Mode for Lithium–Oxygen Batteries

Lithium–oxygen batteries have an ultrahigh theoretical energy density, almost ten times higher than lithium‐ion batteries. The poor conductivity of the discharge product Li₂O₂, however, severely raises the charge overpotential and pulls down the cyclability. Here, a simple and effective strategy is presented for regular formation of lithium vacancies in the discharge product via tuning charge/discharge mode, and their effects on the charge transfer behavior. The effects of the discharge current density on the lithium vacancies, ionic conductivity, and electronic conductivity of the discharge product Li₂O₂ are systematically investigated via electron spin resonance, spin‐alignment echo nuclear magnetic resonance, and tungsten nanomanipulators, respectively. The study by density functional theory indicates that the lithium vacancies in Li₂O₂ generated during the discharge process are highly dependent on the current density. High current can induce a high vacancy density, which enhances the electronic conductivity and reduces the overpotential. Meanwhile, with increasing discharge current, the morphology of the Li₂O₂ changes from microtoroids to thin nanoplatelets, effectively shortening the charge transfer distance and improving the cycling performance. The Li₂O₂ grown in fast discharge mode is more easily decomposed in the following charging process. The lithium–oxygen battery cycling in fast‐discharge/slow‐charge mode exhibits low overpotential and long cycle life.     

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

The aprotic lithium–oxygen (Li–O₂) battery has excited huge interest due to it having the highest theoretical energy density among the different types of rechargeable battery. The facile achievement of a practical Li–O₂ battery has been proven unrealistic, however. The most significant barrier to progress is the limited understanding of the reaction processes occurring in the battery, especially during the charging process on the positive electrode. Thus, understanding the charging mechanism is of crucial importance to enhance the Li–O₂ battery performance and lifetime. Here, recent progress in understanding the electrochemistry and chemistry related to charging in Li–O₂ batteries is reviewed along with the strategies to address the issues that exist in the charging process at the present stage. The properties of Li₂O₂ and the mechanisms of Li₂O₂ oxidation to O₂ on charge are discussed comprehensively, as are the accompanied parasitic chemistries, which are considered as the underlying issues hindering the reversibility of Li–O₂ batteries. Based on the detailed discussion of the charging mechanism, innovative strategies for addressing the issues for the charging process are discussed in detail. This review has profound implications for both a better understanding of charging chemistry and the development of reliable rechargeable Li–O₂ batteries in the future.     

Revisiting the Role of Polysulfides in Lithium–Sulfur Batteries

Intermediate polysulfides (S_n, where n = 2–8) play a critical role in both mechanistic understanding and performance improvement of lithium–sulfur batteries. The rational management of polysulfides is of profound significance for high‐efficiency sulfur electrochemistry. Here, the key roles of polysulfides are discussed, with regard to their status, behavior, and their correspondingimpact on the lithium–sulfur system. Two schools of thoughts for polysulfide management are proposed, their advantages and disadvantages are compared, and future developments are discussed.     

Conductive FeOOH as Multifunctional Interlayer for Superior Lithium–Sulfur Batteries

The commercial course of Li–S batteries (LSBs) is impeded by several severe problems, such as low electrical conductivity of S, Li₂S₂, and Li₂S, considerable volume variation up to 80% during multiphase transformation and severe intermediation lithium polysulfides (LiPSs) shuttle effect. To solve above problems, conductive FeOOH interlayer is designed as an effective trapper and catalyst to accelerate the conversion of LiPSs in LSBs. FeOOH nanorod is effectively affinitive to S that Fe atoms act as Lewis acid sites to capture LiPSs via strong chemical anchoring capability and dispersion interaction. The excellent electrocatalytic effect enables that reduced charging potential barrier and enhanced electron/ion transport is realized on the FeOOH interlayer to promote LiPSs conversion. Significantly, Li₂S oxidation process is improved on the FeOOH interlayer determined as a combination of reduced Li₂S decomposition energy barrier and enhanced Li‐ion transport. Therefore, the multifunctional FeOOH interlayer with conductive and catalytic features show strong chemisorption with LiPSs and accelerated LiPSs redox kinetics. As a result, LSBs with FeOOH interlayer displays high discharge capacity of 1449 mAh g^?1 at 0.05 C and low capacity decay of 0.05% per cycle at 1 C, as well as excellent rate capability (449 mAh g^?1 at 2 C).     

Chemical Immobilization Effect on Lithium Polysulfides for Lithium–Sulfur Batteries

Despite great progress in lithium–sulfur batteries (LSBs), great obstacles still exist to achieve high loading content of sulfur and avoid the loss of active materials due to the dissolution of the intermediate polysulfide products in the electrolyte. Relationships between the intrinsic properties of nanostructured hosts and electrochemical performance of LSBs, especially, the chemical interaction effects on immobilizing polysulfides for LSB cathodes, are discussed in this Review. Moreover, the principle of rational microstructure design for LSB cathode materials with strong chemical interaction adsorbent effects on polysulfides, such as metallic compounds, metal particles, organic polymers, and heteroatom‐doped carbon, is mainly described. According to the chemical immobilizing mechanism of polysulfide on LSB cathodes, three kinds of chemical immobilizing effects, including the strong chemical affinity between polar host and polar polysulfides, the chemical bonding effect between sulfur and the special function groups/atoms, and the catalytic effect on electrochemical reaction kinetics, are thoroughly reviewed. To improve the electrochemical performance and long cycling life‐cycle stability of LSBs, possible solutions and strategies with respect to the rational design of the microstructure of LSB cathodes are comprehensively analyzed.     

Textile Inspired Lithium–Oxygen Battery Cathode with Decoupled Oxygen and Electrolyte Pathways

The lithium–air (Li–O₂) battery has been deemed one of the most promising next‐generation energy‐storage devices due to its ultrahigh energy density. However, in conventional porous carbon–air cathodes, the oxygen gas and electrolyte often compete for transport pathways, which limit battery performance. Here, a novel textile‐based air cathode is developed with a triple‐phase structure to improve overall battery performance. The hierarchical structure of the conductive textile network leads to decoupled pathways for oxygen gas and electrolyte: oxygen flows through the woven mesh while the electrolyte diffuses along the textile fibers. Due to noncompetitive transport, the textile‐based Li–O₂ cathode exhibits a high discharge capacity of 8.6 mAh cm^?2, a low overpotential of 1.15 V, and stable operation exceeding 50 cycles. The textile‐based structure can be applied to a range of applications (fuel cells, water splitting, and redox flow batteries) that involve multiple phase reactions. The reported decoupled transport pathway design also spurs potential toward flexible/wearable Li–O₂ batteries.     

The Regulating Role of Carbon Nanotubes and Graphene in Lithium‐Ion and Lithium–Sulfur Batteries

The ever‐increasing demands for batteries with high energy densities to power the portable electronics with increased power consumption and to advance vehicle electrification and grid energy storage have propelled lithium battery technology to a position of tremendous importance. Carbon nanotubes (CNTs) and graphene, known with many appealing properties, are investigated intensely for improving the performance of lithium‐ion (Li‐ion) and lithium–sulfur (Li–S) batteries. However, a general and objective understanding of their actual role in Li‐ion and Li–S batteries is lacking. It is recognized that CNTs and graphene are not appropriate active lithium storage materials, but are more like a regulator: they do not electrochemically react with lithium ions and electrons, but serve to regulate the lithium storage behavior of a specific electroactive material and increase the range of applications of a lithium battery. First, metrics for the evaluation of lithium batteries are discussed, based on which the regulating role of CNTs and graphene in Li‐ion and Li–S batteries is comprehensively considered from fundamental electrochemical reactions to electrode structure and integral cell design. Finally, perspectives on how CNTs and graphene can further contribute to the development of lithium batteries are presented.     

Recent Advances in Hollow Porous Carbon Materials for Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries are considered as one of the most potential next‐generation rechargeable batteries due to their high theoretical energy density. However, some critical issues, such as low capacity, poor cycling stability, and safety concerns, must be solved before Li–S batteries can be used practically. During the past decade, tremendous efforts have been devoted to the design and synthesis of electrode materials. Benefiting from their tunable structural parameters, hollow porous carbon materials (HPCM) remarkably enhance the performances of both sulfur cathodes and lithium anodes, promoting the development of high‐performance Li–S batteries. Here, together with the templated synthesis of HPCM, recent progresses of Li–S batteries based on HPCM are reviewed. Several important issues in Li–S batteries, including sulfur loading, polysulfide entrapping, and Li metal protection, are discussed, followed by a summary on recent research on HPCM‐based sulfur cathodes, modified separators, and lithium anodes. After the discussion on emerging technical obstacles toward high‐energy Li–S batteries, prospects for the future directions of HPCM research in the field of Li–S batteries are also proposed.     

3D Hollow α‐MnO2 Framework as an Efficient Electrocatalyst for Lithium–Oxygen Batteries

Lithium–oxygen (Li–O₂) batteries are attracting more attention owing to their superior theoretical energy density compared to conventional Li‐ion battery systems. With regards to the catalytically electrochemical reaction on a cathode, the electrocatalyst plays a key role in determining the performance of Li–O₂ batteries. Herein, a new 3D hollow α‐MnO₂ framework (3D α‐MnO₂) with porous wall assembled by hierarchical α‐MnO₂ nanowires is prepared by a template‐induced hydrothermal reaction and subsequent annealing treatment. Such a distinctive structure provides some essential properties for Li–O₂ batteries including the intrinsic high catalytic activity of α‐MnO₂, more catalytic active sites of hierarchical α‐MnO₂ nanowires on 3D framework, continuous hollow network and rich porosity for the storage of discharge product aggregations, and oxygen diffusion. As a consequence, 3D α‐MnO₂ achieves a high specific capacity of 8583 mA h g^?1 at a current density of 100 mA g^?1, a superior rate capacity of 6311 mA h g^?1 at 300 mA g^?1, and a very good cycling stability of 170 cycles at a current density of 200 mA g^?1 with a fixed capacity of 1000 mA h g^?1. Importantly, the presented design strategy of 3D hollow framework in this work could be extended to other catalytic cathode design for Li–O₂ batteries.