Toward Promising Cathode Catalysts for Nonlithium Metal–Oxygen Batteries

The success of Li–air/O₂ batteries has brought extensive attention to the development of various promising non‐Li metal–O₂ batteries, such as Zn–O₂, Al–O₂, Mg–O₂ batteries, etc., which have exhibited unique advantages, such as low production cost, high energy density, and much enhanced safety. The versatile non‐Li metal–O₂ batteries provide a better opportunity for meeting the practical requirements for sustainable energy supplies in various applications. A high‐performance cathode in non‐Li metal–O₂ batteries that can effectively trigger both oxygen reduction and evolution reactions and thus boost the overall battery performance is of great research interest. In this article, a comprehensive review on the development of Li‐free metal–O₂ batteries and particularly focusing on the oxygen catalytic cathodes for both primary and secondary non‐Li metal–O₂ batteries is carefully performed. The current challenges and potential solutions are also outlined and proposed. Through carefully selecting and rationally designing promising catalytic cathodes, a series of non‐Li metal–oxygen batteries toward practical energy storage applications are highly anticipated.     

Rational Design of a Dual‐Function Hybrid Cathode Substrate for Lithium–Sulfur Batteries

A unique 3D hybrid sponge with chemically coupled nickel disulfide‐reduced graphene oxide (NiS₂‐RGO) framework is rationally developed as an effective polysulfide reservoir through a biomolecule‐assisted self‐assembly synthesis. An optimized amount of NiS₂ (≈18 wt%) with porous nanoflower‐like morphology is uniformly in situ grown on the RGO substrate, providing abundant active sites to adsorb and localize polysulfides. The improved polysulfide adsorptivity from sulfiphilic NiS₂ is confirmed by experimental data and first‐principle calculations. Moreover, due to the chemical coupling between NiS₂ and RGO formed during the in situ synthesis, the conductive RGO substrate offers a 3D electron pathway to facilitate charge transfer toward the NiS₂‐polysulfide adsorption interface, triggering a fast redox kinetics of polysulfide conversion and excellent rate performance (C/20–4C). Therefore, the self‐assembled hybrid structure simultaneously promotes static polysulfide‐trapping capability and dynamic polysulfide‐conversion reversibility. As a result, the 3D porous sponge enables a high sulfur content (75 wt%) and a remarkably high sulfur loading (up to 21 mg cm^?2) and areal capacity (up to 16 mAh cm^?2), exceeding most of the reported values in the literature involving either RGO or metal sulfides/other metal compounds (sulfur content of <60 wt% and sulfur loading of <3 mg cm^?2).     

Metal–Air Batteries: From Static to Flow System

As an emerging battery technology, metal–air flow batteries inherit the advantageous features of the unique structural design of conventional redox flow batteries and the high energy density of metal–air batteries, thus showing great potential as efficient electrochemical systems for large‐scale electrical energy storage. This review summarizes the operating principles and recent progress of metal–air flow batteries from a materials and chemistry perspective, with particular emphasis on the latest advanced materials design and cell configuration engineering, which the authors divide into three categories based on the anode species: vanadium–air, zinc–air, and lithium–air flow batteries. Since some of the capabilities developed for metal–air static batteries can be leveraged for next‐generation flow systems, classical works on conventional metal–air batteries are selected and compared with the metal–air flow systems, highlighting the prominent advantages of the latter in achieving high energy capacity and long cycle performance. At the end, a general perspective on current challenges/opportunities and future research directions to promote the commercial application of the metal–air flow battery technology is provided. The aim is to provide a comprehensive overview and to set up a road map for guiding development from conventional static to advanced flow technologies of metal–air batteries.     

Interpenetrating Triphase Cobalt‐Based Nanocomposites as Efficient Bifunctional Oxygen Electrocatalysts for Long‐Lasting Rechargeable Zn–Air Batteries

Rational construction of atomic‐scale interfaces in multiphase nanocomposites is an intriguing and challenging approach to developing advanced catalysts for both oxygen reduction (ORR) and evolution reactions (OER). Herein, a hybrid of interpenetrating metallic Co and spinel Co₃O₄ “Janus” nanoparticles stitched in porous graphitized shells (Co/Co₃O₄@PGS) is synthesized via ionic exchange and redox between Co²⁺ and 2D metal–organic‐framework nanosheets. This strategy is proven to effectively establish highways for the transfer of electrons and reactants within the hybrid through interfacial engineering. Specifically, the phase interpenetration of mixed Co species and encapsulating porous graphitized shells provides an optimal charge/mass transport environment. Furthermore, the defect‐rich interfaces act as atomic‐traps to achieve exceptional adsorption capability for oxygen reactants. Finally, robust coupling between Co and N through intimate covalent bonds prohibits the detachment of nanoparticles. As a result, Co/Co₃O₄@PGS outperforms state‐of‐the‐art noble‐metal catalysts with a positive half‐wave potential of 0.89 V for ORR and a low potential of 1.58 V at 10 mA cm^?2 for OER. In a practical demonstration, ultrastable cyclability with a record lifetime of over 800 h at 10 mA cm^?2 is achieved by Zn–air batteries with Co/Co₃O₄@PGS within the rechargeable air electrode.     

Metal–Organic Frameworks‐Derived Tunnel Structured Co3(PO4)2@C as Cathode for New Generation High‐Performance Al‐Ion Batteries

Despite great progress in aluminum ion batteries (AIBs), the commercialization and performance improvement of AIBs‐based carbon cathodes is greatly impeded by sluggish intercalation/extraction and redox kinetics due to large‐sized AlCl₄^? anions. Phosphates with tunnel channels and much larger d‐spacing than the radius of Al³⁺ could be an alternative candidate as a cathode for potential high‐performance AIBs. Herein, elaborately designed porous tunnel structured Co₃(PO₄)₂@C composites derived from ZIF‐67 as AIBs cathodes are demonstrated, showing increased active sites, high ionic mobility, and high Al³⁺ ion diffusion coefficient, leading to remarkably enhanced discharge–charge redox reaction kinetics. Furthermore, the carbon shell and porous structure performs as armor to alleviate volume change and maintain the structure integrity of the cathodes. As expected, the rationally constructed Co₃(PO₄)₂@C composite exhibits a superior capacity of 111 mA h g^?1 at a high current density of 6 A g^?1 and 151 mA h g^?1 at 2 A g^?1 after 500 cycles with capacity decay of 0.02% per cycle. This innovative strategy could be a big step forward for long‐term cycle stable AIBs and reveals significant insights into the redox reaction mechanism for high‐performance AIBs based on Al³⁺ rather than large‐sized AlCl₄^?.     

Trifunctional Electrocatalysis on Dual‐Doped Graphene Nanorings–Integrated Boxes for Efficient Water Splitting and Zn–Air Batteries

Despite the exciting achievements made in synthesis of monofunctional electrocatalysts for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), or hydrogen evolution reaction (HER), it is challenging to develop trifunctional electrocatalysts for both ORR/OER/HER. Herein, N, O‐codoped graphene nanorings‐integrated boxes (denoted NOGB) are crafted via high‐temperature pyrolysis and following acid etching of hybrid precursors containing polymers and Prussian blue analogue cubes. The electrochemical results signified that the resulting NOGB‐800 (800 refers to pyrolysis temperature) is highly active for trifunctional electrocatalysis of ORR/OER/HER. This can be reasonably attributed to the advanced nanostructures (i.e., the hierarchically porous nanostructures on the hollow nanorings) and unique chemical compositions (i.e., N, O‐codoped graphene). More attractively, the rechargeable Zn–air battery based on NOGB‐800 displays maximum power density of 111.9 mW cm^?2 with small charge–discharge potential of 0.72 V and excellent stability of 30 h, comparable with the Pt/C+Ir/C counterpart. The NOGB‐800 could also be utilized as bifunctional electrocatalysts for overall water splitting to yield current density of 10 mA cm^?2 at a voltage of 1.65 V, surpassing most reported electrocatalysts. Therefore, the NOGB‐800 is a promising candidate instead of precious metal–based electrocatalysts for the efficient Zn–air battery and water splitting.     

Strategies Toward Stable Nonaqueous Alkali Metal–O2 Batteries

Alkali metal–O₂ batteries, by coupling high‐capacity alkali metal anodes with gaseous oxygen, possess extremely high gravimetric energy density that is comparable to gasoline and are potential energy storage technologies beyond lithium–ion batteries. The development of alkali metal–O₂ batteries has achieved great progress in recent years, from materials to prototype devices and on fundamental mechanisms. The stability of alkali metal–O₂ batteries is still poor, however, leading to a huge gap between laboratory research and commercial applications. A series of parasitic reactions result in the instability, which occur during electrochemical discharging and charging. The ubiquitous active oxygen species attack both the organic electrolyte and the carbon cathode, triggering various parasitic reactions. Meanwhile, dendrite growth and volume expansion upon repeated plating/stripping and O₂ crossover severely limit the reversibility of alkali metal anodes. Here, an overview of the strategies against these issues is given to improve the stability of nonaqueous alkali metal–O₂ batteries, which is discussed from three aspects: air cathodes, alkali metal anodes, and aprotic electrolytes. Furthermore, perspectives for future research of stable alkali metal–O₂ batteries are outlined.     

Super‐Stretchable Zinc–Air Batteries Based on an Alkaline‐Tolerant Dual‐Network Hydrogel Electrolyte

Stretchable devices need elastic hydrogel electrolyte as an essential component, while most hydrogels will lose their stretchability after being incorporated with strong alkaline solution. This is why highly stretchable zinc–air batteries have never been reported so far. Herein, super‐stretchable, flat‐ (800% stretchable) and fiber‐shaped (500% stretchable) zinc–air batteries are first developed by designing an alkaline‐tolerant dual‐network hydrogel electrolyte. In the dual‐network hydrogel electrolyte, sodium polyacrylate (PANa) chains contribute to the formation of soft domains and the carboxyl groups neutralized by hydroxyls as well as cellulose as potassium hydroxide stabilizer are responsible for vastly enhanced alkaline tolerance. The obtained super‐stretchable, flat zinc–air battery exhibits a high power density of 108.6 mW?cm^?2, increasing to 210.5 mW?cm^?2 upon being 800% stretched. Similar phenomena are observed for the 500% stretchable fiber‐shaped batteries. The devices can maintain stable power output even after being heavily deformed benefiting from the highly soft, alkaline‐tolerant hydrogel electrolyte developed. A bendable battery‐display system and water proof weavable fiber zinc–air battery are also demonstrated. This work will facilitate the progress of using zinc–air battery powering flexible electronics and smart clothes. Moreover, the developed alkaline‐tolerant super‐stretchable electrolyte can also be applied for many other alkaline electrolyte‐based energy storage/conversion devices.     

Hierarchical NiCo2S4@NiO Core–Shell Heterostructures as Catalytic Cathode for Long‐Life Li‐O2 Batteries

The critical challenges of Li‐O₂ batteries lie in sluggish oxygen redox kinetics and undesirable parasitic reactions during the oxygen reduction reaction and oxygen evolution reaction processes, inducing large overpotential and inferior cycle stability. Herein, an elaborately designed 3D hierarchical heterostructure comprising NiCo₂S₄@NiO core–shell arrays on conductive carbon paper is first reported as a freestanding cathode for Li‐O₂ batteries. The unique hierarchical array structures can build up multidimensional channels for oxygen diffusion and electrolyte impregnation. A built‐in interfacial potential between NiCo₂S₄ and NiO can drastically enhance interfacial charge transfer kinetics. According to density functional theory calculations, intrinsic LiO₂‐affinity characteristics of NiCo₂S₄ and NiO play an importantly synergistic role in promoting the formation of large peasecod‐like Li₂O₂, conducive to construct a low‐impedance Li₂O₂/cathode contact interface. As expected, Li‐O₂ cells based on NiCo₂S₄@NiO electrode exhibit an improved overpotential of 0.88 V, a high discharge capacity of 10 050 mAh g^?1 at 200 mA g^?1, an excellent rate capability of 6150 mAh g^?1 at 1.0 A g^?1, and a long‐term cycle stability under a restricted capacity of 1000 mAh g^?1 at 200 mA g^?1. Notably, the reported strategy about heterostructure accouplement may pave a new avenue for the effective electrocatalyst design for Li‐O₂ batteries.     

Metal‐Organic Framework Integrated Anodes for Aqueous Zinc‐Ion Batteries

Zinc‐based batteries have a high capacity and are safe, cost‐effective, environmentally‐friendly, and capable of scalable production. However, dendrite formation and poor reversibility hinder their performance. Metal‐organic framework (MOF)‐based Zn anodes are made by wet chemistry to address these issues. These MOF‐based anodes exhibit high efficiency during Zn plating‐stripping and prevent dendrite formation, as shown by ex situ SEM analysis. The practicality of the MOF‐based anodes is demonstrated in aqueous Zn ion batteries, which show improved performance including specific capacity, cycle life, and safety relative to the pristine Zn anode due to their hydrophilic and porous surface. These results, along with the easy scalability of the process, demonstrate the high potential of MOF‐modified Zn anodes for use in dendrite‐free, higher‐performance, Zn‐based energy storage systems.     

Atomic Modulation of FeCo–Nitrogen–Carbon Bifunctional Oxygen Electrodes for Rechargeable and Flexible All‐Solid‐State Zinc–Air Battery

Rational design and exploration of robust and low‐cost bifunctional oxygen reduction/evolution electrocatalysts are greatly desired for metal–air batteries. Herein, a novel high‐performance oxygen electrode catalyst is developed based on bimetal FeCo nanoparticles encapsulated in in situ grown nitrogen‐doped graphitic carbon nanotubes with bamboo‐like structure. The obtained catalyst exhibits a positive half‐wave potential of 0.92 V (vs the reversible hydrogen electrode, RHE) for oxygen reduction reaction, and a low operating potential of 1.73 V to achieve a 10 mA cm^?2 current density for oxygen evolution reaction. The reversible oxygen electrode index is 0.81 V, surpassing that of most highly active bifunctional catalysts reported to date. By combining experimental and simulation studies, a strong synergetic coupling between FeCo alloy and N‐doped carbon nanotubes is proposed in producing a favorable local coordination environment and electronic structure, which affords the pyridinic N‐rich catalyst surface promoting the reversible oxygen reactions. Impressively, the assembled zinc–air batteries using liquid electrolytes and the all‐solid‐state batteries with the synthesized bifunctional catalyst as the air electrode demonstrate superior charging–discharging performance, long lifetime, and high flexibility, holding great potential in practical implementation of new‐generation powerful rechargeable batteries with portable or even wearable characteristic.     

Metal‐Air Batteries: Metal–Air Batteries with High Energy Density: Li–Air versus Zn–Air (Adv. Energy Mater. 1/2011)

In the past decade, there have been exciting developments in the field of lithium ion batteries as energy storage devices, resulting in the application of lithium ion batteries in areas ranging from small portable electric devices to large power systems such as hybrid electric vehicles. However, the maximum energy density of current lithium ion batteries having topatactic chemistry is not sufficient to meet the demands of new markets in such areas as electric vehicles. Therefore, new electrochemical systems with higher energy densities are being sought, and metal‐air batteries with conversion chemistry are considered a promising candidate. More recently, promising electrochemical performance has driven much research interest in Li‐air and Zn‐air batteries. This review provides an overview of the fundamentals and recent progress in the area of Li‐air and Zn‐air batteries, with the aim of providing a better understanding of the new electrochemical systems.     

Soft,Highly Elastic,and Discharge‐Current‐Controllable Eutectic Gallium–Indium Liquid Metal–Air Battery Operated at Room Temperature

Based on a liquid metal (eutectic alloy with 90 wt% gallium and 10 wt% indium) anode, a soft, highly elastic, discharge‐current‐controllable, cable‐shaped liquid metal–air battery operated at 25 °C, with effective reactions of Ga ? 3e^? → Ga³⁺ and O₂ + 2H₂O + 4e^? → 4OH^? is presented. In the liquid metal electrode, indium is used not only to inhibit the corrosion of gallium in the alkaline electrolyte but also to maintain the liquid state of the anode at room temperature. Thus, the liquid anode can be easily injected into (or extracted from) the battery cavity, leading to an easily renewable anode. In addition, the cable‐shaped battery shows a pressure‐responsive discharge current, owing to the soft, deformable battery body. Due to the liquid anode and flexible carbon fiber‐based cathode, the battery is highly flexible (bending radius < 1 mm) and easily recovers from any degree of bending without electrochemical performance impairment. With its elastic polyacrylic acid‐based gel electrolyte, the battery shows high elasticity, stretching by up to 100% (from 12 to 24 cm), excellent shape recovery from stretched states, and a discharge performance retention of 98.87%. Moreover, this paper provides the possibility to develop a deformable battery based on the liquid metal material.     

Metal–Air Batteries with High Energy Density: Li–Air versus Zn–Air

In the past decade, there have been exciting developments in the field of lithium ion batteries as energy storage devices, resulting in the application of lithium ion batteries in areas ranging from small portable electric devices to large power systems such as hybrid electric vehicles. However, the maximum energy density of current lithium ion batteries having topatactic chemistry is not sufficient to meet the demands of new markets in such areas as electric vehicles. Therefore, new electrochemical systems with higher energy densities are being sought, and metal‐air batteries with conversion chemistry are considered a promising candidate. More recently, promising electrochemical performance has driven much research interest in Li‐air and Zn‐air batteries. This review provides an overview of the fundamentals and recent progress in the area of Li‐air and Zn‐air batteries, with the aim of providing a better understanding of the new electrochemical systems.