Mixed Transition‐Metal Oxides: Design,Synthesis, and Energy‐Related Applications

A promising family of mixed transition‐metal oxides (MTMOs) (designated as A_xB_3‐xO₄; A, B=Co, Ni, Zn, Mn, Fe, etc.) with stoichiometric or even non‐stoichiometric compositions, typically in a spinel structure, has recently attracted increasing research interest worldwide. Benefiting from their remarkable electrochemical properties, these MTMOs will play significant roles for low‐cost and environmentally friendly energy storage/conversion technologies. In this Review, we summarize recent research advances in the rational design and efficient synthesis of MTMOs with controlled shapes, sizes, compositions, and micro‐/nanostructures, along with their applications as electrode materials for lithium‐ion batteries and electrochemical capacitors, and efficient electrocatalysts for the oxygen reduction reaction in metal–air batteries and fuel cells. Some future trends and prospects to further develop advanced MTMOs for next‐generation electrochemical energy storage/conversion systems are also presented.     

A Bimetallic Zn/Fe Polyphthalocyanine‐Derived Single‐Atom Fe‐N4 Catalytic Site:A Superior Trifunctional Catalyst for Overall Water Splitting and Zn–Air Batteries

Developing an efficient single‐atom material (SAM) synthesis and exploring the energy‐related catalytic reaction are important but still challenging. A polymerization–pyrolysis–evaporation (PPE) strategy was developed to synthesize N‐doped porous carbon (NPC) with anchored atomically dispersed Fe‐N₄ catalytic sites. This material was derived from predesigned bimetallic Zn/Fe polyphthalocyanine. Experiments and calculations demonstrate the formed Fe‐N₄ site exhibits superior trifunctional electrocatalytic performance for oxygen reduction, oxygen evolution, and hydrogen evolution reactions. In overall water splitting and rechargeable Zn–air battery devices containing the Fe‐N₄ SAs/NPC catalyst, it exhibits high efficiency and extraordinary stability. This current PPE method is a general strategy for preparing M SAs/NPC (M=Co, Ni, Mn), bringing new perspectives for designing various SAMs for catalytic application.     

Photo‐excited Oxygen Reduction and Oxygen Evolution Reactions Enable a High‐Performance Zn–Air Battery

The storage of solar energy in battery systems is pivotal for a sustainable society, which faces many challenges. Herein, a Zn–air battery is constructed with two cathodes of poly(1,4‐di(2‐thienyl))benzene (PDTB) and TiO₂ grown on carbon papers to sandwich a Zn anode. The PDTB cathode is illuminated in a discharging process, in which photoelectrons are excited into the conduction band of PDTB to promote oxygen reduction reaction (ORR) and raise the output voltage. In a reverse process, holes in the valence band of the illuminated TiO₂ cathode are driven for the oxygen evolution reaction (OER) by an applied voltage. A record‐high discharge voltage of 1.90 V and an unprecedented low charge voltage of 0.59 V are achieved in the photo‐involved Zn–air battery, regardless of the equilibrium voltage. This work offers an innovative pathway for photo‐energy utilization in rechargeable batteries.     

Atomically Dispersed Iron–Nitrogen Species as Electrocatalysts for Bifunctional Oxygen Evolution and Reduction Reactions

Rational design of non‐noble materials as highly efficient, economical, and durable bifunctional catalysts for oxygen evolution and reduction reactions (OER/ORR) is currently a critical obstacle for rechargeable metal‐air batteries. A new route involving S was developed to achieve atomic dispersion of Fe‐N_x species on N and S co‐decorated hierarchical carbon layers, resulting in single‐atom bifunctional OER/ORR catalysts for the first time. The abundant atomically dispersed Fe‐N_x species are highly catalytically active, the hierarchical structure offers more opportunities for active sites, and the electrical conductivity is greatly improved. The obtained electrocatalyst exhibits higher limiting current density and a more positive half‐wave potential for ORR, as well as a lower overpotential for OER under alkaline conditions. Moreover, a rechargeable Zn–air battery device comprising this hybrid catalyst shows superior performance compared to Pt/C catalyst. This work will open a new avenue to design advanced bifunctional catalysts for reversible energy storage and conversion devices.     

The Proportion of Fe‐NX,N Doping Species and Fe3C to Oxygen Catalytic Activity in Core‐Shell Fe‐N/C Electrocatalyst

A bifunctional oxygen electrocatalyst composed of iron carbide (Fe₃C) nanoparticles encapsulated by nitrogen doped carbon sheets is reported. X‐ray photoelectron spectroscopy and X‐ray absorption near edge structure revealed the presence of several kinds of active sites (Fe?N_x sites, N doping sites) and the modulated electron structure of nitrogen doped carbon sheets. Fe₃C@N‐CSs shows excellent oxygen evolution and oxygen reduction catalytic activity owing to the modulated electron structure by encapsulated Fe₃C core via biphasic interfaces electron interaction, which can lower the free energy of intermediate, strengthen the bonding strength and enhance conductivity. Meanwhile, the contribution of the Fe?N_x sites, N doping sites and the effect of Fe₃C core for the electrocatalytic oxygen reaction is originally revealed. The Fe₃C@N‐CSs air electrode‐based zinc‐air battery demonstrates a high open circuit potential of 1.47 V, superior charge‐discharge performance and long lifetime, which outperforms the noble metal‐based zinc‐air battery.     

A Rechargeable Al/S Battery with an Ionic‐Liquid Electrolyte

Aluminum metal is a promising anode material for next generation rechargeable batteries owing to its abundance, potentially dendrite‐free deposition, and high capacity. The rechargeable aluminum/sulfur (Al/S) battery is of great interest owing to its high energy density (1340 Wh kg^?1) and low cost. However, Al/S chemistry suffers poor reversibility owing to the difficulty of oxidizing AlS_x. Herein, we demonstrate the first reversible Al/S battery in ionic‐liquid electrolyte with an activated carbon cloth/sulfur composite cathode. Electrochemical, spectroscopic, and microscopic results suggest that sulfur undergoes a solid‐state conversion reaction in the electrolyte. Kinetics analysis identifies that the slow solid‐state sulfur conversion reaction causes large voltage hysteresis and limits the energy efficiency of the system.     

Interacting Carbon Nitride and Titanium Carbide Nanosheets for High‐Performance Oxygen Evolution

Free‐standing flexible films, constructed from two‐dimensional graphitic carbon nitride and titanium carbide (with MXene phase) nanosheets, display outstanding activity and stability in catalyzing the oxygen‐evolution reaction in alkaline aqueous system, which originates from the Ti–N_x motifs acting as electroactive sites, and the hierarchically porous structure with highly hydrophilic surface. With this excellent electrocatalytic ability, comparable to that of the state‐of‐the‐art precious‐/transition‐metal catalysts and superior to that of most free‐standing films reported to date, they are directly used as efficient cathodes in rechargeable zinc–air batteries. Our findings reveal that the rational interaction between different two‐dimensional materials can remarkably promote the oxygen electrochemistry, thus boosting the entire clean energy system.     

Pomegranate‐Inspired Design of Highly Active and Durable Bifunctional Electrocatalysts for Rechargeable Metal–Air Batteries

Rational design of highly active and durable electrocatalysts for oxygen reactions is critical for rechargeable metal–air batteries. Herein, we report the design and development of composite electrocatalysts based on transition metal oxide nanocrystals embedded in a nitrogen‐doped, partially graphitized carbon framework. Benefiting from the unique pomegranate‐like architecture, the composite catalysts possess abundant active sites, strong synergetic coupling, enhanced electron transfer, and high efficiencies in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The Co₃O₄‐based composite electrocatalyst exhibited a high half‐wave potential of 0.842 V for ORR, and a low overpotential of only 450 mV at the current density of 10 mA cm^?2 for OER. A single‐cell zinc–air battery was also fabricated with superior durability, holding great promise in the practical implementation of rechargeable metal–air batteries.     

Zinc‐Reduced Mesoporous TiOx Li‐Ion Battery Anodes with Exceptional Rate Capability and Cycling Stability

We demonstrate a unique synthetic route for oxygen‐deficient mesoporous TiO_x by a redox–transmetalation process by using Zn metal as the reducing agent. The as‐obtained materials have significantly enhanced electronic conductivity; 20 times higher than that of as‐synthesized TiO₂ material. Moreover, electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) measurements are performed to validate the low charge carrier resistance of the oxygen‐deficient TiO_x. The resulting oxygen‐deficient TiO_x battery anode exhibits a high reversible capacity (～180 mA h g^?1 at a discharge/charge rate of 1 C/1 C after 400 cycles) and an excellent rate capability (～90 mA h g^?1 even at a rate of 10 C). Also, the full cell, which is coupled with a LiCoO₂ cathode material, exhibits an outstanding rate capability (>75 mA h g^?1 at a rate of 3.0 C) and maintains a reversible capacity of over 100 mA h g^?1 at a discharge/charge of 1 C/1 C for 300 cycles.     

Recycling Application of Li–MnO2 Batteries as Rechargeable Lithium–Air Batteries

The ever‐increasing consumption of a huge quantity of lithium batteries, for example, Li–MnO₂ cells, raises critical concern about their recycling. We demonstrate herein that decayed Li–MnO₂ cells can be further utilized as rechargeable lithium–air cells with admitted oxygen. We further investigated the effects of lithiated manganese dioxide on the electrocatalytic properties of oxygen‐reduction and oxygen‐evolution reactions (ORR/OER). The catalytic activity was found to be correlated with the composition of Li_xMnO₂ electrodes (0<x<1) generated in situ in aprotic Li–MnO₂ cells owing to tuning of the Mn valence and electronic structure. In particular, modestly lithiated Li_0.50MnO₂ exhibited superior performance with enhanced round‐trip efficiency (ca. 76 %), high cycling ability (190 cycles), and high discharge capacity (10 823 mA h g_carbon^?1). The results indicate that the use of depleted Li–MnO₂ batteries can be prolonged by their application as rechargeable lithium–air batteries.     

Sulfur‐Based Electrodes that Function via Multielectron Reactions for Room‐Temperature Sodium‐Ion Storage

Emerging rechargeable sodium‐ion storage systems—sodium‐ion and room‐temperature sodium–sulfur (RT‐NaS) batteries—are gaining extensive research interest as low‐cost options for large‐scale energy‐storage applications. Owing to their abundance, easy accessibility, and unique physical and chemical properties, sulfur‐based materials, in particular metal sulfides (MS_x) and elemental sulfur (S), are currently regarded as promising electrode candidates for Na‐storage technologies with high capacity and excellent redox reversibility based on multielectron conversion reactions. Here, we present current understanding of Na‐storage mechanisms of the S‐based electrode materials. Recent progress and strategies for improving electronic conductivity and tolerating volume variations of the MS_x anodes in Na‐ion batteries are reviewed. In addition, current advances on S cathodes in RT‐NaS batteries are presented. We outline a novel emerging concept of integrating MS_x electrocatalysts into conventional carbonaceous matrices as effective polarized S hosts in RT‐NaS batteries as well. This comprehensive progress report could provide guidance for research toward the development of S‐based materials for the future Na‐storage techniques.     

Molecular Engineering of a 3D Self‐Supported Electrode for Oxygen Electrocatalysis in Neutral Media

Electrodes for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are required in energy conversion and storage technologies. An assembly strategy involves covalently grafting Co corrole 1 onto Fe₃O₄ nanoarrays grown on Ti mesh. The resulted electrode shows significantly improved activity and durability for OER and ORR in neutral media as compared to Fe₃O₄ alone and with directly adsorbed 1 . It also displays higher atom efficiency (at least two magnitudes larger turnover frequency) than reported electrodes. Using this electrode in a neutral Zn‐air battery, a small charge–discharge voltage gap of 1.19 V, large peak power density of 90.4 mW cm^?2, and high rechargeable stability for >100 h are achieved, opening a promising avenue of molecular electrocatalysis in a metal–air battery. This work shows a molecule‐engineered electrode for electrocatalysis and demonstrates their potential applications in energy conversion and storage.     

From K‐O2 to K‐Air Batteries: Realizing Superoxide Batteries on the Basis of Dry Ambient Air

Although using an air cathode is the goal for superoxide‐based potassium‐oxygen (K‐O₂) batteries, prior studies were limited to pure oxygen. Now, the first K‐air (dry) battery based on reversible superoxide electrochemistry is presented. Spectroscopic and gas chromatography analyses are applied to evaluate the reactivity of KO₂ in ambient air. Although KO₂ reacts with water vapor and CO₂ to form KHCO₃, it is highly stable in dry air. With this knowledge, rechargeable K‐air (dry) batteries were successfully demonstrated by employing dry air cathode. The reduced partial pressure of oxygen plays a critical role in boosting battery lifespan. With a more stable environment for the K anode, a K‐air (dry) battery delivers over 100 cycles (>500 h) with low round‐trip overpotentials and high coulombic efficiencies as opposed to traditional K‐O₂ battery that fails early. This work sheds light on the benefits and restrictions of employing the air cathode in superoxide‐based batteries.     

Engineering Fast Ion Conduction and Selective Cation Channels for a High‐Rate and High‐Voltage Hybrid Aqueous Battery

The rechargeable aqueous metal‐ion battery (RAMB) has attracted considerable attention due to its safety, low costs, and environmental friendliness. Yet the poor‐performance electrode materials lead to a low feasibility of practical application. A hybrid aqueous battery (HAB) built from electrode materials with selective cation channels could increase the electrode applicability and thus enlarge the application of RAMB. Herein, we construct a high‐voltage K–Na HAB based on K₂FeFe(CN)₆ cathode and carbon‐coated NaTi₂(PO₄)₃ (NTP/C) anode. Due to the unique cation selectivity of both materials and ultrafast ion conduction of NTP/C, the hybrid battery delivers a high capacity of 160 mAh g^?1 at a 0.5 C rate. Considerable capacity retention of 94.3 % is also obtained after 1000 cycles at even 60 C rate. Meanwhile, high energy density of 69.6 Wh kg^?1 based on the total mass of active electrode materials is obtained, which is comparable and even superior to that of the lead acid, Ni/Cd, and Ni/MH batteries.     

Metal‐containing Ionic Liquid/Polyacrylonitrile‐derived Carbon Nanofibers for Oxygen Reduction Reaction and Flexible Zn–Air Battery

Practical applications of Zn–air batteries are usually limited by sluggish kinetics of oxygen reduction reaction. Replacing Pt‐based catalysts with convenient, efficient and low‐cost materials to boost oxygen reduction reaction is highly desirable. Herein, a class of Fe?N co‐doped carbon nanofibers is successfully synthesized by pyrolysis of polyacrylonitrile/metal‐containing ionic liquid‐based electrospun films. The ionic liquids act as porogen to provide multiscale pores as well as activator to bring carbon nanofibers active sites. The catalyst possessing appropriate active sites and unique 3D porous architecture exhibits remarkable long‐term stability and electrocatalytic activity. Particularly, the catalyst maintains a shape of membrane after carbonization, manifesting its direct use as air electrode without binders. It is notable that an all solid‐state Zn–air battery based on the carbon nanofibers exhibits good flexibility, indicating its promising application as wearable devices.     

High‐Performance K–CO2 Batteries Based on Metal‐Free Carbon Electrocatalysts

Metal–CO₂ batteries have attracted much attention owing to their high energy density and use of greenhouse CO₂ waste as the energy source. However, the increasing cost of lithium and the low discharge potential of Na–CO₂ batteries create obstacles for practical applications of Li/Na–CO₂ batteries. Recently, earth‐abundant potassium ions have attracted considerable interest as fast ionic charge carriers for electrochemical energy storage. Herein, we report the first K–CO₂ battery with a carbon‐based metal‐free electrocatalyst. The battery shows a higher theoretical discharge potential (E^?=2.48 V) than that of Na–CO₂ batteries (E^?=2.35 V) and can operate for more than 250 cycles (1500 h) with a cutoff capacity of 300 mA h g^?1. Combined DFT calculations and experimental observations revealed a reaction mechanism involving the reversible formation and decomposition of P12₁/c1‐type K₂CO₃ at the efficient carbon‐based catalyst.     

Efforts towards Practical and Sustainable Li/Na‐Air Batteries

The Li‐O₂ batteries have attracted much attention due to their parallel theoretical energy density to gasoline. In the past 20 years, understanding and knowledge in Li‐O₂ battery have greatly deepened in elucidating the relationship between structure and performance. Our group has been focusing on the cathode engineering and anode protection strategy development in the past years, trying to make full use of the superiority of metal‐air batteries towards applications. In this review, we aim to retrospect our efforts in developing practical, sustainable metal‐air batteries. We will first introduce the basic working principle of Li‐O₂ batteries and our progresses in Li‐O₂ batteries with typical cathode designs and anode protection strategies, which have together promoted the large capacity, long life and low charge overpotential. We emphasize the designing art of carbon‐based cathodes in this part along with a short talk on all‐metal cathodes. The following part is our research in Na‐O₂ batteries including both cathode and anode optimizations. The differences between Li‐O₂ and Na‐O₂ batteries are also briefly discussed. Subsequently, our proof‐of‐concept work on Li‐N₂ battery, a new energy storage system and chemistry, is discussed with detailed information on the discharge product identification. Finally, we summarize our designed models and prototypes of flexible metal‐air batteries that are promising to be used in flexible devices to deliver more power.     

Photo-excited Oxygen Reduction and Oxygen Evolution Reactions Enable a High-Performance Zn–Air Battery

The storage of solar energy in battery systems is pivotal for a sustainable society, which faces many challenges. Herein, a Zn–air battery is constructed with two cathodes of poly(1,4-di(2-thienyl))benzene (PDTB) and TiO₂ grown on carbon papers to sandwich a Zn anode. The PDTB cathode is illuminated in a discharging process, in which photoelectrons are excited into the conduction band of PDTB to promote oxygen reduction reaction (ORR) and raise the output voltage. In a reverse process, holes in the valence band of the illuminated TiO₂ cathode are driven for the oxygen evolution reaction (OER) by an applied voltage. A record-high discharge voltage of 1.90 V and an unprecedented low charge voltage of 0.59 V are achieved in the photo-involved Zn–air battery, regardless of the equilibrium voltage. This work offers an innovative pathway for photo-energy utilization in rechargeable batteries.     

Double‐Helix Structure in Carrageenan–Metal Hydrogels: A General Approach to Porous Metal Sulfides/Carbon Aerogels with Excellent Sodium‐Ion Storage

The metal sulfide‐carbon nanocomposite is a new class of anode material for sodium ion batteries, but its development is restricted by its relative poor rate ability and cyclic stability. Herein, we report the use of double‐helix structure of carrageenan–metal hydrogels for the synthesis of 3D metal sulfide (M_xS_y) nanostructure/carbon aerogels (CAs) for high‐performance sodium‐ion storage. The method is unique, and can be used to make multiple M_xS_y/CAs (such as FeS/CA, Co₉S₈/CA, Ni₃S₄/CA, CuS/CA, ZnS/CA, and CdS/CA) with ultra‐small nanoparticles and hierarchical porous structure by pyrolyzing the carrageenan–metal hydrogels. The as‐prepared FeS/CA exhibits a high reversible capacity and excellent cycling stability (280 mA h^?1 at 0.5 A g^?1 over 200 cycles) and rate performance (222 mA h^?1 at 5 A g^?1) when used as the anode material for sodium‐ion batteries. The work shows the value of biomass‐derived metal sulfide–carbon heterostuctures in sodium‐ion storage.     

A Liquid/Liquid Electrolyte Interface that Inhibits Corrosion and Dendrite Growth of Lithium in Lithium‐Metal Batteries

A proof‐of‐concept study on a liquid/liquid (L/L) two‐phase electrolyte interface is reported by using the polarity difference of solvent for the protection of Li‐metal anode with long‐term operation over 2000 h. The L/L electrolyte interface constructed by non‐polar fluorosilicane (PFTOS) and conventionally polar dimethyl sulfoxide solvents can block direct contact between conventional electrolyte and Li anode, and consequently their side reactions can be significantly eliminated. Moreover, the homogeneous Li‐ion flow and Li‐mass deposition can be realized by the formation of a thin and uniform solid‐electrolyte interphase (SEI) composed of LiF, Li_xC, Li_xSiO_y between PFTOS and Li anode, as well as the super‐wettability state of PFTOS to Li anode, resulting in the suppression of Li dendrite formation. The cycling stability in a lithium–oxygen battery as a model is improved 4 times with the L/L electrolyte interface.