Oxygen Reduction Kinetics of Fe-N-C Single Atom Catalysts Boosted by Pyridinic N Vacancy for Temperature-Adaptive Zn-Air Batteries.

The design of temperature-adaptive Zn-air batteries (ZABs) with long life spans and high energy efficiencies is challenging owing to sluggish oxygen reduction reaction (ORR) kinetics and an unstable Zn/electrolyte interface. Herein, a quasi-solid-state ZAB is designed by combining atomically dispersed Fe-N-C catalysts containing pyridinic N vacancies (FeNC-VN) with a polarized organo-hydrogel electrolyte. First-principles calculation predicts that adjacent VN sites effectively enhance the covalency of Fe-Nx moieties and moderately weaken *OH binding energies, significantly boosting the ORR kinetics and stability. In situ Raman spectra reveal the dynamic evolution of *O2- and *OOH on the FeNC-VN cathode in the aqueous ZAB, proving that the 4e- associative mechanism is dominant. Moreover, the ethylene glycol-modulated organo-hydrogel electrolyte forms a zincophilic protective layer on the Zn anode surface and tailors the [Zn(H2O)6]2+ solvation sheath, effectively guiding epitaxial deposition of Zn2+ on the Zn (002) plane and suppressing side reactions. The assembled quasi-solid-state ZAB demonstrates a long life span of over 1076 h at 2 mA cm-2 at -20 °C, outperforming most reported ZABs.

Heterogeneous Catalyst as a Functional Substrate Governing the Shape of Electrochemical Precipitates in Oxygen-Fueled Rechargeable Batteries.

Lithium-oxygen batteries have the potential to become the most eminent solution for future energy storage with their theoretical energy density exceeding all existing batteries. However, the insulating and insoluble discharge product (lithium peroxide; Li2O2) impairs practical application. Conventional catalyst designs based on the electronic structure and interfacial charge transfer descriptors have not been able to overcome these limitations due to Li2O2. Herein, we revisit the role of heterogeneous catalysts as substrates to regulate Li2O2 growth and the formation of solid/solid reaction interfaces. We demonstrate that controlled solid/solid interfacial structure design is a critical performance parameter beyond the inherent electronic structure. In particular, the Cu2O substrate in this study induces a homogeneous deposition of Pd atoms, which leads to well-controlled growth of Li2O2 resolving mass and charge transport limits (i.e., the bottleneck of oxygen reduction/evolution reactions), thus improving reversibility, capacity, and durability of the cells by dissipating electrochemical and mechanical stress. We thus verified the essential role of solid/solid interfaces to regulate the nucleation and growth process of Li2O2 in lithium-oxygen batteries.

Regulating Pseudo-Jahn-Teller Effect and Superstructure in Layered Cathode Materials for Reversible Alkali-Ion Intercalation.

The Jahn-Teller effect (JTE) is one of the most important determinators of how much stress layered cathode materials undergo during charge and discharge; however, many reports have shown that traces of superstructure exist in pristine layered materials and irreversible phase transitions occur even after eliminating the JTE. A careful consideration of the energy of cationic distortion using a Taylor expansion indicated that second-order JTE (pseudo-JTE) is more widespread than the aforementioned JTE because of the various bonding states that occur between bonding and antibonding molecular orbitals in transition-metal octahedra. As a model case, a P2-type Mn-rich cathode (Na3/4MnO2) was investigated in detail. MnO6 octahedra are well known to undergo either elongation or contraction in a specific direction due to JTE. Here, the substitution of Li for Mn (Na3/4(Li1/4Mn3/4)O2) helped to oxidize Mn3+ to Mn4+ suppressing JTE; however, the MnO6 octahedra remained asymmetric with a clear trace of the superstructure. With various advanced analyses, we disclose the pseudo-JTE as a general reason for the asymmetric distortions of the MnO6 octahedra. These distortions lead to the significant electrochemical degradation of Na3/4Li1/4Mn3/4O2. The suppression of the pseudo-JTE modulates phase transition behaviors during Na intercalation/deintercalation and thereby improves all of the electrochemical properties. The insight obtained by coupling a theoretical background for the pseudo-JTE with verified layered cathode material lattice changes implies that many previous approaches can be rationalized by regulating pseudo-JTE. This suggests that the pseudo-JTE should be thought more important than the well-known JTE for layered cathode materials.

Activating a Multielectron Reaction of NASICON-Structured Cathodes toward High Energy Density for Sodium-Ion Batteries.

The increasing demand to efficiently store and utilize the electricity from renewable energy resources in a sustainable way has boosted the request for sodium-ion battery technology due to the high abundance of sodium sources worldwide. Na superionic conductor (NASICON) structured cathodes with a robust polyanionic framework have been intriguing because of their open 3D structure and superior thermal stability. The ever-increasing demand for higher energy densities with NASICON-structured cathodes motivates us to activate multielectron reactions, thus utilizing the third sodium ion toward higher voltage and larger capacity, both of which have been the bottlenecks for commercializing sodium-ion batteries. A doping strategy with Cr inspired by first-principles calculations enables the activation of multielectron redox reactions of the redox couples V2+/V3+, V3+/V4+, and V4+/V5+, resulting in remarkably improved energy density even in comparison to the layer structured oxides and Prussian blue analogues. This work also comprehensively clarifies the role of the Cr dopant during sodium storage and the valence electron transition process of both V and Cr. Our findings highlight the importance of a broadly applicable doping strategy for achieving multielectron reactions of NASICON-type cathodes with higher energy densities in sodium-ion batteries.

Synergistic Control of Structural Disorder and Surface Bonding Nature to Optimize the Functionality of Manganese Oxide as an Electrocatalyst and a Cathode for Li-O2 Batteries.

An efficient way to improve the electrocatalyst and Li-O2 battery performances of metal oxide is developed by an exquisite synergistic control over structural disorder and surface bonding nature. The effects of amorphous nature and surface chemical environment on the functionalities of metal oxide are systematically investigated with well-crystalline and amorphous MnO2 nanocrystals with/without surface anchoring of highly oxidized iodate clusters. The amorphous MnO2 nanocrystal containing anchored iodate clusters shows much better performance as an oxygen evolution electrocatalyst and cathode catalyst for Li-O2 batteries than both iodate-free amorphous and well-crystalline homologues, underscoring the remarkable advantage of simultaneous enhancement of structural disorder and surface electron density. In situ X-ray absorption spectroscopic analysis demonstrates the promoted formation of double (MnO) bond, a critical step of oxygen evolution reaction, upon amorphization caused by the poor orbital overlap inside highly disordered crystallites. The beneficial effects of iodate anchoring and amorphization on electrocatalyst functionality are attributable to the alteration of surface bonding character, stabilization of Jahn-Teller active Mn3+ species, and enhanced charge transfer of interfaces. The present study underscores that fine-tuning of structural disorder and surface bonding nature provides an effective methodology to explore efficient metal oxide-based electrocatalysts.

Reversible Anionic Redox Activities in Conventional LiNi1/3 Co1/3 Mn1/3 O2 Cathodes.

Redox reactions of oxygen have been considered critical in controlling the electrochemical properties of lithium-excessive layered-oxide electrodes. However, conventional electrode materials without overlithiation remain the most practical. Typically, cationic redox reactions are believed to dominate the electrochemical processes in conventional electrodes. Herein, we show unambiguous evidence of reversible anionic redox reactions in LiNi1/3 Co1/3 Mn1/3 O2 . The typical involvement of oxygen through hybridization with transition metals is discussed, as well as the intrinsic oxygen redox process at high potentials, which is 75 % reversible during initial cycling and 63 % retained after 10 cycles. Our results clarify the reaction mechanism at high potentials in conventional layered electrodes involving both cationic and anionic reactions and indicate the potential of utilizing reversible oxygen redox reactions in conventional layered oxides for high-capacity lithium-ion batteries.

Advances in the Cathode Materials for Lithium Rechargeable Batteries.

The accelerating development of technologies requires a significant energy consumption, and consequently the demand for advanced energy storage devices is increasing at a high rate. In the last two decades, lithium-ion batteries have been the most robust technology, supplying high energy and power density. Improving cathode materials is one of the ways to satisfy the need for even better batteries. Therefore developing new types of positive electrode materials by increasing cell voltage and capacity with stability is the best way towards the next-generation Li rechargeable batteries. To achieve this goal, understanding the principles of the materials and recognizing the problems confronting the state-of-the-art cathode materials are essential prerequisites. This Review presents various high-energy cathode materials which can be used to build next-generation lithium-ion batteries. It includes nickel and lithium-rich layered oxide materials, high voltage spinel oxides, polyanion, cation disordered rock-salt oxides and conversion materials. Particular emphasis is given to the general reaction and degradation mechanisms during the operation as well as the main challenges and strategies to overcome the drawbacks of these materials.

Uncovering the Shuttle Effect in Organic Batteries and Counter-Strategies Thereof: A Case Study of the N,N'-Dimethylphenazine Cathode.

The main drawback of organic electrode materials is their solubility in the electrolyte, leading to the shuttle effect. Using N,N'-dimethylphenazine (DMPZ) as a highly soluble cathode material, and its PF6 - and triflimide salts as models for its first oxidation state, a poor correlation was found between solubility and battery operability. Extensive electrochemical experiments suggest that the shuttle effect is unlikely to be mediated by molecular diffusion as commonly understood, but rather by electron-hopping via the electron self-exchange reaction based on spectroscopic results. These findings led to two counter-strategies to prevent the hopping process: the pre-treatment of the anode to form a solid-electrolyte interface and using DMPZ salt rather than neutral DMPZ as the active material. These strategies improved coulombic efficiency and capacity retention, demonstrating that solubility of organic materials does not necessarily exclude their applications in batteries.

Large-Scale Synthesis of MOF-Derived Superporous Carbon Aerogels with Extraordinary Adsorption Capacity for Organic Solvents.

Carbon aerogels (CAs) with 3D interconnected networks hold promise for application in areas such as pollutant treatment, energy storage, and electrocatalysis. In spite of this, it remains challenging to synthesize high-performance CAs on a large scale in a simple and sustainable manner. We report an eco-friendly method for the scalable synthesis of ultralight and superporous CAs by using cheap and widely available agarose (AG) biomass as the carbon precursor. Zeolitic imidazolate framework-8 (ZIF-8) with high porosity is introduced into the AG aerogels to increase the specific surface area and enable heteroatom doping. After pyrolysis under inert atmosphere, the ZIF-8/AG-derived nitrogen-doped CAs show a highly interconnected porous mazelike structure with a low density of 24 mg cm-3 , a high specific surface area of 516 m2 g-1 , and a large pore volume of 0.58 cm-3 g-1 . The resulting CAs exhibit significant potential for application in the adsorption of organic pollutants.

Universal Access to Two-Dimensional Mesoporous Heterostructures by Micelle-Directed Interfacial Assembly.

Two-dimensional (2D) mesoporous heterostructures combining ultrathin nanosheet morphology, periodic porous surface structures, and diverse hybrid compositions have become increasingly important for renewable energy storage and electronics. However, it remains a great challenge to develop a universal method to prepare 2D mesoporous heterostructures. Herein, we report a composite-micelle-directed interfacial assembly method to synthesize heterostructures of an ultrathin 2D material covered with mesoporous monolayers assembled on both sides. To demonstrate the concept, we first fabricated a new sandwichlike carbon@MXene@carbon mesoporous heterostructure through the self-assembly of exfoliated MXene nanosheets and block copolymer F127/melamine-formaldehyde resin composite micelles and subsequent thermal treatment. Finally, we demonstrate that the carbon@MXene@carbon mesoporous heterostructured nanosheets manifest remarkably enhanced electrochemical performance as a cathode material for lithium-sulfur batteries.

Chemical Design of Palladium-Based Nanoarchitectures for Catalytic Applications.

Palladium (Pd) plays an important role in numerous catalytic reactions, such as methanol and ethanol oxidation, oxygen reduction, hydrogenation, coupling reactions, and carbon monoxide oxidation. Creating Pd-based nanoarchitectures with increased active surface sites, higher density of low-coordinated atoms, and maximized surface coverage for the reactants is important. To address the limitations of pure Pd, various Pd-based nanoarchitectures, including alloys, intermetallics, and supported Pd nanomaterials, have been fabricated by combining Pd with other elements with similar or higher catalytic activity for many catalytic reactions. Herein, recent advances in the preparation of Pd-based nanoarchitectures through solution-phase chemical reduction and electrochemical deposition methods are summarized. Finally, the trend and future outlook in the development of Pd nanocatalysts toward practical catalytic applications are discussed.

Template-Free Fabrication of Mesoporous Alumina Nanospheres Using Post-Synthesis Water-Ethanol Treatment of Monodispersed Aluminium Glycerate Nanospheres for Molybdenum Adsorption.

This work reports the template-free fabrication of mesoporous Al2 O3 nanospheres with greatly enhanced textural characteristics through a newly developed post-synthesis "water-ethanol" treatment of aluminium glycerate nanospheres followed by high temperature calcination. The proposed "water-ethanol" treatment is highly advantageous as the resulting mesoporous Al2 O3 nanospheres exhibit 2-4 times higher surface area (up to 251 m2 g-1 ), narrower pore size distribution, and significantly lower crystallization temperature than those obtained without any post-synthesis treatment. To demonstrate the generality of the proposed strategy, a nearly identical post-synthesis "water treatment" method is successfully used to prepare mesoporous monometallic (e.g., manganese oxide (MnO2 )) and bimetallic oxide (e.g., CuCo2 O4 and MnCo2 O4 ) nanospheres assembled of nanosheets or nanoplates with highly enhanced textural characteristics from the corresponding monometallic and bimetallic glycerate nanospheres, respectively. When evaluated as molybdenum (Mo) adsorbents for potential use in molybdenum-99/technetium-99m (99 Mo/99m Tc) generators, the treated mesoporous Al2 O3 nanospheres display higher molybdenum adsorption performance than non-treated Al2 O3 nanospheres and commercial Al2 O3 , thereby suggesting the effectiveness of the proposed strategy for improving the functional performance of oxide materials. It is expected that the proposed method can be utilized to prepare other mesoporous metal oxides with enhanced textural characteristics and functional performance.

Spatially Confined Assembly of Monodisperse Ruthenium Nanoclusters in a Hierarchically Ordered Carbon Electrode for Efficient Hydrogen Evolution.

The redox units of polyaniline (PAni) are used cooperatively, and in situ, to assemble ruthenium (Ru) nanoclusters in a hierarchically ordered carbon electrode. The oxidized quinonoid imine (QI) units in PAni bond Ru complex ions selectively, whereas reduced benzenoid amine (BA) units cannot. By electrochemically tuning the ratio of QI to BA, Ru complexes are spatially confined in the outer layer of hierarchical PAni frameworks. Carbonization of Ru-PAni hybrids induces nucleation on the outer surface of the carbon support, generating nearly monodisperse Ru nanoclusters. The optimized catalyst has a low loading of approximately 2 wt % Ru, but exhibits a mass activity for the hydrogen evolution reaction that is about 6.8 times better than commercial 20 wt % Pt/C catalyst.

α-MnO2 Nanowire-Anchored Highly Oxidized Cluster as a Catalyst for Li-O2 Batteries: Superior Electrocatalytic Activity and High Functionality.

An effective chemical way to optimize the oxygen electrocatalyst and Li-O2 electrode functionalities of metal oxide can be developed by the control of chemical bond nature with the surface anchoring of highly oxidized selenate (SeO4 2- ) clusters. The bond competition between (Se6+ -O) and (Mn-O) bonds is quite effective in stabilizing Jahn-Teller-active Mn3+ state and in increasing oxygen electron density of α-MnO2 nanowire (NW). The selenate-anchored α-MnO2 NW shows excellent oxygen electrocatalytic activity and electrode performance for Li-O2 batteries, which is due to the improved charge transfer kinetics and reversible formation/decomposition of Li2 O2 . The present study underscores that the surface anchoring of highly oxidized cluster can provide a facile, effective way of improving the oxygen electrocatalyst and electrochemical performances of nanostructured metal oxide in Li-O2 cells.

In situ analyses for ion storage materials.

Development of high performance electrode materials for energy storage is one of the most important issues for our future society. However, a lack of clear analytical views limits critical understanding of electrode materials. This review covers useful analytical work using X-ray diffraction, X-ray absorption spectroscopy, microscopy and neutron diffraction for ion storage systems. The in situ observation facilitates comprehending real-time ion storage behaviour while the ion storage system is operating, which help us to understand detailed physical and chemical properties. We will discuss how the tools have been used to reveal detailed reaction mechanisms and underlying properties of electrode materials.

SnS 3D Flowers with Superb Kinetic Properties for Anodic Use in Next-Generation Sodium Rechargeable Batteries.

Tin sulfide (SnS) 3D flowers containing hierarchical nanosheet subunits are synthesized using a simple polyol process. The Li ion cells incorporating SnS 3D flowers exhibit an excellent rate capability, as well as good cycling stability, compared to SnS bulks and Sn nanoparticles. These desirable properties can be attributed to their unique morphology having not only large surface reaction area but also enough space between individual 2D nanosheets, which alleviates the pulverization of SnS.

Ordered Mesoporous Cobalt Phosphate with Crystallized Walls toward Highly Active Water Oxidation Electrocatalysts.

A hexagonally ordered mesoporous cobalt phosphate (CoPi) material is prepared by a facile one-pot soft-templating strategy using cetyltrimethylammonium bromide template. Because of its highly accessible surface area and crystalline framework with abundant active sites, the mesoporous CoPi shows a high catalytic activity for the oxygen evolution reaction compared to previously reported noble/transition-metal and nonmetal catalysts.

Graphite-Nanoplate-Coated Bi2 S3 Composite with High-Volume Energy Density and Excellent Cycle Life for Room-Temperature Sodium-Sulfide Batteries.

Graphite-nanoplate-coated Bi2 S3 composite (Bi2 S3 @C) has been prepared by a simple, scalable, and energy-efficient precipitation method combined with ball milling. The Bi2 S3 @C composite was used as the cathode material for sodium-sulfide batteries. It delivered an initial capacity of 550 mAh g(-1) and high stable specific energy in the range of 275-300 Wh kg(-1) at 0.1 C, with an enhanced capacity retention of 69 % over 100 cycles. The unique structure demonstrates superior cycling stability, with a capacity drop of 0.3 % per cycle over 100 cycles, compared with that of bare Bi2 S3 . The sodium storage mechanism of Bi2 S3 was investigated based on ex situ X-ray diffraction and scanning transmission electron microscopy.

Cobalt-Doped FeS2 Nanospheres with Complete Solid Solubility as a High-Performance Anode Material for Sodium-Ion Batteries.

Considering that the high capacity, long-term cycle life, and high-rate capability of anode materials for sodium-ion batteries (SIBs) is a bottleneck currently, a series of Co-doped FeS2 solid solutions with different Co contents were prepared by a facile solvothermal method, and for the first time their Na-storage properties were investigated. The optimized Co0.5 Fe0.5 S2 (Fe0.5) has discharge capacities of 0.220 Ah g(-1) after 5000 cycles at 2 A g(-1) and 0.172 Ah g(-1) even at 20 A g(-1) with compatible ether-based electrolyte in a voltage window of 0.8-2.9 V. The Fe0.5 sample transforms to layered Nax Co0.5 Fe0.5 S2 by initial activation, and the layered structure is maintained during following cycles. The redox reactions of Nax Co0.5 Fe0.5 S2 are dominated by pseudocapacitive behavior, leading to fast Na(+) insertion/extraction and durable cycle life. A Na3 V2 (PO4 )3 /Fe0.5 full cell was assembled, delivering an initial capacity of 0.340 Ah g(-1) .