Calcium-Pillar Boosting Smooth Phase Transition in Potassium Vanadate Nanobelts toward Superior Cycling Performance in Potassium-Ion Batteries.

The depletion of lithium resources has prompted exploration into alternative rechargeable energy storage systems, and potassium-ion batteries (PIBs) have emerged as promising candidates. As an active cathode material for PIBs, potassium vanadate (KxV2O5) usually suffers from structural damage during electrochemical K-ion insertion/extraction and hence leading to unsatisfactory cycling performance. Here, we introduce Ca2+ ions as pillars into the potassium vanadate to enhance its structural stability and smooth its phase transition behavior. The additional Ca2+ not only stabilizes the layered structure but also promotes the rearrangement of interlayer ions and leads to a smooth solid-solution phase transition. The optimal composition K0.36Ca0.05V2O5 (KCVO) exhibits outstanding cyclic stability, delivering a capacity of ∼90 mA h g-1 at 20 mA g-1 with negligible capacity decay even after 700 cycles at 500 mA g-1. Theoretical calculations indicate lower energy barriers for K+ diffusion, promoting rapid reaction kinetics. The excellent performances and detailed investigations offer insights into the structural regulation of layered vanadium cathodes.

Electronically Temperature-Dependent Interplay between He and Trivacancy in Tungsten Plasma-Facing Materials.

Both microvoids and helium (He) impurities are widely present in tungsten (W) plasma-facing materials (PFMs), where the interaction between microvoids and He atoms has led to the intriguing development of microvoids. In this paper, we comprehensively investigated the interaction between He atoms and trivacancy (V3), a fundamental microvoid in W-PFMs, at the level of tight-binding theory. Our study showed that He atoms can catalyze the decomposition of the original V3 or facilitate its transformation into another V3 variant. We propose that a He atom near the V3 defect induces significant changes in the distribution of d-electron charges within the W atoms lining the inner wall of the V3 defect, making the W atom nearest to this He atom cationic and the other W atoms anionic. The attractive interaction between them promotes the decomposition and deformation of V3. As electronic excitation increases, the ionization of W atoms on the V3 wall gradually intensifies, thereby enhancing the cationic characteristics of the W atoms closest to the He atom. This process also prompts other W atoms to shift from anions to cations, leading to a transition in the electrostatic interactions between them from attraction to repulsion. This transformation, driven by electronic excitation, plays a significant inhibitory role in the decomposition and deformation of V3.

Electron transfer bridge inducing polarization of nitrogen molecules for enhanced photocatalytic nitrogen fixation.

Ammonia (NH3) plays a crucial role in the production of fertilizers, medicines, fibers, etc., which are closely relevant to the development of human society. However, the inert and nonpolar properties of NN seriously hinder artificial nitrogen fixation under mild conditions. Herein, we introduce a novel strategy to enhance the photocatalytic efficiency of N2 fixation through the directional polarization of N2 by rare earth metal atoms, which act as a local "electron transfer bridge." This bridge facilitates the transfer of delocalized electrons to the distal N atom and redirects the polarization of adsorbed N2 molecules. Taking cerium doped BiOCl (Ce-BiOCl) as an example, our results reveal that the electrons transfer to the distal N atom through the cerium atom, resulting in absorbed nitrogen molecular polarization. Consequently, the polarized nitrogen molecules exhibit an easier trend for NN cleavage and the subsequent hydrogenation process, and exhibit a greatly enhanced photocatalytic ammonia production rate of 46.7 µmol g-1 h-1 in cerium doped BiOCl, nearly 4 times higher than that of pure BiOCl. The original concept of directional polarization of N2 presented in this work not only deepens our understanding of the N2 molecular activation mechanism but also broadens our horizons for designing highly efficient catalysts for N2 fixation.

Novel tungsten nitride crystal providing nanochannels for hydrogen removal and recycling in PFMs.

Hydrogen (H) removal in plasma-facing materials (PFMs) has been an important issue in the field of manually controllable fusion reactions. The tungsten nitride (WNx) film, as a by-product on the divertor surface in the fusion reactor after nitrogen seeding, has rich H retention, and much attention should be given to hydrogen removal from WNx. In this paper, by using density functional theory calculations, we predicted a novel W24N48 crystal, which possesses nanoscale cavities and channels inside, and studied the interaction between it and hydrogen. We found that the N atoms inside the crystal are favorable for the adsorption of atomic hydrogen. When more hydrogen atoms are injected, the hydrogen atoms adsorbed in the crystal react with the newly entered hydrogen atoms to form hydrogen molecules. These newly formed H2 molecules can easily move through the nanochannels in W24N48 and serve to remove hydrogen. Our calculations suggest that when this new W24N48 material is embedded in the WNx film on the surface of the PFMs, the nanochannels will be helpful in removing and recycling hydrogen isotopes in the PFMs.

Introducing a Pseudocapacitive Lithium Storage Mechanism into Graphite by Defect Engineering for Fast-Charging Lithium-Ion Batteries.

The extreme fast-charging capability of lithium-ion batteries (LIBs) is very essential for electric vehicles (EVs). However, currently used graphite anode materials cannot satisfy the requirements of fast charging. Herein, we demonstrate that intrinsic lattice defect engineering based on a thermal treatment of graphite in CO2 is an effective method to improve the fast-charging capability of the graphite anode. The activated graphite (AG) exhibits a superior rate capability of 209 mAh g-1 at 10 C (in comparison to 15 mAh g-1 for the pristine graphite), which is attributed to a pseudocapacitive lithium storage behavior. Furthermore, the full cell LiFePO4||AG can achieve SOCs of 82% and 96% within 6 and 15 min, respectively. The intrinsic carbon defect introduced by the CO2 treatment succeeds in improving the kinetics of lithium ion intercalation at the rate-determining step during lithiation, which is identified by the distribution of relaxation times (DRT) and density functional theory (DFT) calculations. Therefore, this study provides a novel strategy for fast-charging LIBs. Moreover, this facile method is also suitable for activating other carbon-based materials.

Active-Site-Specific Structural Engineering Enabled Ultrahigh Rate Performance of the NaLi3Fe3(PO4)2(P2O7) Cathode for Lithium-Ion Batteries.

Iron-based mixed-polyanionic cathode Na4Fe3(PO4)2(P2O7) (NFPP) has advantages of environmental benignity, easy synthesis, high theoretical capacity, and remarkable stability. From NFPP, a novel Li-replaced material NaLi3Fe3(PO4)2(P2O7) (NLFPP) is synthesized through active Na-site structural engineering by an electrochemical ion exchange approach. The NLFPP cathode can show high reversible capacities of 103.2 and 90.3 mA h g-1 at 0.5 and 5C, respectively. It also displays an impressive discharge capacity of 81.5 mA h g-1 at an ultrahigh rate of 30C. Density functional theory (DFT) calculation demonstrates that the formation energy of NLFPP is the lowest among NLFPP, NFPP, and NaFe3(PO4)2(P2O7), indicating that NLFPP is the easiest to form and the conversion from NFPP to NLFPP is thermodynamically favorable. The Li substitution for Na in the NFPP lattice causes an increase in the unit cell parameter c and decreases in a, b, and V, which are revealed by both DFT calculations and in situ X-ray powder diffraction (XRD) analysis. With hard carbon (HC) as the anode, the NLFPP//HC full cell shows a high reversible capacity of 91.1 mA h g-1 at 2C and retains 82.4% after 200 cycles. The proposed active-site-specific structural tailoring via electrochemical ion exchange will give new insights into the design of high-performance cathodes for lithium-ion batteries.

High ICE Hard Carbon Anodes for Lithium-Ion Batteries Enabled by a High Work Function.

Hard carbons (HC) derived from biomass material are most promising anodes for lithium-ion batteries (LIBs) because of their cost effectiveness and environmental friendliness. However, the low initial Coulombic efficiency (ICE) of HC anodes reduces the energy density of full cells, which seriously impedes their practical applications. Herein, we demonstrate that the ICE of HC anodes can be significantly improved by modulating the work function of a model HC derived from cotton and deliberately treated to form C-Cl bonds on its surface. By X-ray absorption near-edge structure and density functional theory (DFT) calculation studies, it is verified that the introduction of the C-Cl bond leads to the electron transfer from C to Cl and enhances the work function of the system. In addition, this Cl-doped HC anode can inhibit the reduction of solvent molecules in the electrolyte and reduce the formation of a solid electrolyte interface (SEI) film. Consequently, the ICE is improved from 64.8 to 78.1%. This study provides an effective route to reduce the formation of the SEI film and improve the ICE of hard carbon anodes for LIBs.

Thermal Conductivity, Electrical Resistivity, and Microstructure of Cu/W Multilayered Nanofilms.

Metallic multilayered nanofilms have been extensively studied owing to their unique physical properties and applications. However, studies on the thermal conductivity and electrical resistivity of metallic multilayered nanofilms, as their important physical properties, are seldom reported. In this work, Cu/W multilayered nanofilms with periodic thickness varying from 6 to 150 nm were deposited by magnetron sputtering. The resistivities of the Cu/W multilayered nanofilms increase with the decrease of periodic thickness, especially when the periodic thickness is smaller than 37 nm. The resistivities of the multilayered nanofilms fit well with the Fuchs-Sondheimer and Mayadas-Shatzkes (FS-MS) model, which considers both interface scattering and grain boundary scattering. The thermal conductivities of the Cu/W multilayered nanofilms were measured by the three-omega (3ω) method, which decrease with a decrease of periodic thickness initially and increase at the smallest periodic thickness of 6 nm. The Boltzmann transport equation (BTE)-based model was used, to explain the periodic thickness-dependent thermal conductivity of metallic multilayered nanofilms by considering the contributions from both phonon and electron heat transport processes, where the calculated thermal conductivities agree well with the measured ones. The electrical resistivity and thermal conductivity strongly depend on the microstructures of the multilayered nanofilms.

Effect of cation replacement on the phase stability of formamidinium lead iodide perovskite.

First-principles calculations have been performed to study the effect of cation replacement with methylammonium (MA+), Cs+, and Rb+ on the properties of formamidinium lead iodide (FAPbI3) perovskite. It is found that these dopants could improve the stability of the desired α phase of FAPbI3 at reduced temperature by lowering the transition temperature between the perovskite cubic α phase and nonperovskite hexagonal δ phase. Interestingly, the optical absorption properties and the effective masses of holes of FAPbI3 perovskite are only slightly affected. The nature of the improvement of the phase stability resulting from the cation replacement is revealed. However, the calculated mixing energies indicate that these multication materials still suffer long-term instability. Our results provide theoretical guidance for improving current multication engineering strategies or even developing new approaches.

Intercalated Iridium Diselenide Electrocatalysts for Efficient pH-Universal Water Splitting.

Developing bifunctional catalysts for both hydrogen and oxygen evolution reactions is a promising approach to the practical implementation of electrocatalytic water splitting. However, most of the reported bifunctional catalysts are only applicable to alkaline electrolyzer, although a few are effective in acidic or neutral media that appeals more to industrial applications. Here, a lithium-intercalated iridium diselenide (Li-IrSe2 ) is developed that outperformed other reported catalysts toward overall water splitting in both acidic and neutral environments. Li intercalation activated the inert pristine IrSe2 via bringing high porosities and abundant Se vacancies for efficient hydrogen and oxygen evolution reactions. When Li-IrSe2 was assembled into two-electrode electrolyzers for overall water splitting, the cell voltages at 10 mA cm-2 were 1.44 and 1.50 V under pH 0 and 7, respectively, being record-low values in both conditions.

Oxygen vacancy associated single-electron transfer for photofixation of CO2 to long-chain chemicals.

The photofixation and utilization of CO2 via single-electron mechanism is considered to be a clean and green way to produce high-value-added commodity chemicals with long carbon chains. However, this topic has not been fully explored for the highly negative reduction potential in the formation of reactive carbonate radical. Herein, by taking Bi2O3 nanosheets as a model system, we illustrate that oxygen vacancies confined in atomic layers can lower the adsorption energy of CO2 on the reactive sites, and thus activate CO2 by single-electron transfer in mild conditions. As demonstrated, Bi2O3 nanosheets with rich oxygen vacancies show enhanced generation of •CO2- species during the reaction process and achieve a high conversion yield of dimethyl carbonate (DMC) with nearly 100% selectivity in the presence of methanol. This study establishes a practical way for the photofixation of CO2 to long-chain chemicals via defect engineering.

Facilitating Lithium-Ion Diffusion in Layered Cathode Materials by Introducing Li+/Ni2+ Antisite Defects for High-Rate Li-Ion Batteries.

Li+/Ni2+ antisite defects mainly resulting from their similar ionic radii in the layered nickel-rich cathode materials belong to one of cation disordering scenarios. They are commonly considered harmful to the electrochemical properties, so a minimum degree of cation disordering is usually desired. However, this study indicates that LiNi0.8Co0.15Al0.05O2 as the key material for Tesla batteries possesses the highest rate capability when there is a minor degree (2.3%) of Li+/Ni2+ antisite defects existing in its layered structure. By combining a theoretical calculation, the improvement mechanism is attributed to two effects to decrease the activation barrier for lithium migration: (1) the anchoring of a low fraction of high-valence Ni2+ ions in the Li slab pushes uphill the nearest Li+ ions and (2) the same fraction of low-valence Li+ ions in the Ni slab weakens the repulsive interaction to the Li+ ions at the saddle point.

Pothole-rich Ultrathin WO3 Nanosheets that Trigger N≡N Bond Activation of Nitrogen for Direct Nitrate Photosynthesis.

Nitrate is a raw ingredient for the production of fertilizer, gunpowder, and explosives. Developing an alternative approach to activate the N≡N bond of naturally abundant nitrogen to form nitrate under ambient conditions will be of importance. Herein, pothole-rich WO3 was used to catalyse the activation of N≡N covalent triple bonds for the direct nitrate synthesis at room temperature. The pothole-rich structure endues the WO3 nanosheet more dangling bonds and more easily excited high momentum electrons, which overcome the two major bottlenecks in N≡N bond activation, that is, poor binding of N2 to catalytic materials and the high energy involved in this reaction. The average rate of nitrate production is as high as 1.92 mg g-1 h-1 under ambient conditions, without any sacrificial agent or precious-metal co-catalysts. More generally, the concepts will initiate a new pathway for triggering inert catalytic reactions.

Oxygen-Vacancy-Mediated Exciton Dissociation in BiOBr for Boosting Charge-Carrier-Involved Molecular Oxygen Activation.

Excitonic effects mediated by Coulomb interactions between photogenerated electrons and holes play crucial roles in photoinduced processes of semiconductors. In terms of photocatalysis, however, efforts have seldom been devoted to the relevant aspects. For the catalysts with giant excitonic effects, the coexisting, competitive exciton generation serves as a key obstacle to the yield of free charge carriers, and hence, transformation of excitons into free carriers would be beneficial for optimizing the charge-carrier-involved photocatalytic processes. Herein, by taking bismuth oxybromide (BiOBr) as a prototypical model system, we demonstrate that excitons can be effectively dissociated into charge carriers with the incorporation of oxygen vacancy, leading to excellent performances in charge-carrier-involved photocatalytic reactions such as superoxide generation and selective organic syntheses under visible-light illumination. This work not only establishes an in-depth understanding of defective structures in photocatalysts but also paves the way for excitonic regulation via defect engineering.

Highly Active Fe Sites in Ultrathin Pyrrhotite Fe7S8 Nanosheets Realizing Efficient Electrocatalytic Oxygen Evolution.

Identification of active sites in an electrocatalyst is essential for understanding of the mechanism of electrocatalytic water splitting. To be one of the most active oxygen evolution reaction catalysts in alkaline media, Ni-Fe based compounds have attracted tremendous attention, while the role of Ni and Fe sites played has still come under debate. Herein, by taking the pyrrhotite Fe7S8 nanosheets with mixed-valence states and metallic conductivity for examples, we illustrate that Fe could be a highly active site for electrocatalytic oxygen evolution. It is shown that the delocalized electrons in the ultrathin Fe7S8 nanosheets could facilitate electron transfer processes of the system, where d orbitals of FeII and FeIII would be overlapped with each other during the catalytic reactions, rendering the ultrathin Fe7S8 nanosheets to be the most efficient Fe-based electrocatalyst for water oxidation. As expected, the ultrathin Fe7S8 nanosheets exhibit promising electrocatalytic oxygen evolution activities, with a low overpotential of 0.27 V and a large current density of 300 mA cm-2 at 0.5 V. This work provides solid evidence that Fe could be an efficient active site for electrocatalytic water splitting.

Conductive Tungsten Oxide Nanosheets for Highly Efficient Hydrogen Evolution.

Exploring efficient and economical electrocatalysts for hydrogen evolution reaction is of great significance for water splitting on an industrial scale. Tungsten oxide, WO3, has been long expected to be a promising non-precious-metal electrocatalyst for hydrogen production. However, the poor intrinsic activity of this material hampers its development. Herein, we design a highly efficient hydrogen evolution electrocatalyst via introducing oxygen vacancies into WO3 nanosheets. Our first-principles calculations demonstrate that the gap states introduced by O vacancies make WO3 act as a degenerate semiconductor with high conductivity and desirable hydrogen adsorption free energy. Experimentally, we prepared WO3 nanosheets rich in oxygen vacancies via a liquid exfoliation, which indeed exhibits the typical character of a degenerate semiconductor. When evaluated by hydrogen evolution, the nanosheets display superior performance with a small overpotential of 38 mV at 10 mA cm-2 and a low Tafel slope of 38 mV dec-1. This work opens an effective route to develop conductive tungsten oxide as a potential alternative to the state-of-the-art platinum for hydrogen evolution.

Local Electric Field Facilitates High-Performance Li-Ion Batteries.

By scrutinizing the energy storage process in Li-ion batteries, tuning Li-ion migration behavior by atomic level tailoring will unlock great potential for pursuing higher electrochemical performance. Vacancy, which can effectively modulate the electrical ordering on the nanoscale, even in tiny concentrations, will provide tempting opportunities for manipulating Li-ion migratory behavior. Herein, taking CuGeO3 as a model, oxygen vacancies obtained by reducing the thickness dimension down to the atomic scale are introduced in this work. As the Li-ion storage progresses, the imbalanced charge distribution emerging around the oxygen vacancies could induce a local built-in electric field, which will accelerate the ions' migration rate by Coulomb forces and thus have benefits for high-rate performance. Furthermore, the thus-obtained CuGeO3 ultrathin nanosheets (CGOUNs)/graphene van der Waals heterojunctions are used as anodes in Li-ion batteries, which deliver a reversible specific capacity of 1295 mAh g-1 at 100 mA g-1, with improved rate capability and cycling performance compared to their bulk counterpart. Our findings build a clear connection between the atomic/defect/electronic structure and intrinsic properties for designing high-efficiency electrode materials.

Delocalized Spin States in 2D Atomic Layers Realizing Enhanced Electrocatalytic Oxygen Evolution.

The electrocatalytic activity of transition-metal-based compounds is strongly related to the spin states of metal atoms. However, the ways for regulation of spin states of catalysts are still limited, and the underlying relationship between the spin states and catalytic activities remains unclear. Herein, for the first time, by taking NiII -based compounds without high or low spin states for example, it is shown that their spin states can be delocalized after introducing structural distortion to the atomic layers. The delocalized spin states for Ni atoms can provide not only high electrical conductivity but also low adsorption energy between the active sites and reaction intermediates for the system. As expected, the ultrathin nanosheets of nickel-chalcogenides with structural distortions show dramatically enhanced activity in electrocatalytic oxygen evolution compared to their corresponding bulk samples. This work establishes new way for the design of advanced electrocatalysts in transition-metal-based compounds via regulation of spin states.

Invariant wide bandgaps in honeycomb monolayer and single-walled nanotubes of IIB-VI semiconductors.

The search for low-dimensional materials with unique electronic properties is important for the development of electronic devices in the nanoscale. Through systematic first-principles calculations, we found that the band gaps of the two-dimensional honeycomb monolayers (HMs) and one-dimensional single-walled nanotubes (SWNTs) of IIB-VI semiconductors (ZnO, CdO, ZnS and CdS) are nearly chirality-independent and weakly diameter-dependent. Based on analysis of the electronic structures, it was found that the conduction band minimum is contributed to by the spherically symmetric s orbitals of cations and the valence band maximum is dominated by the in-plane [Formula: see text] and [Formula: see text] hybridizations. These electronic states are robust against radius curvature, resulting in the invariant feature of the band gaps for the structures changing from HM to SWNTs. The band gaps of these materials range from 2.3 to 4.7 eV, which is of potential application in electronic devices and optoelectronic devices. Our studies show that searching for and designing specific electronic structures can facilitate the process of exploring novel nanomaterials for future applications.