Construction of N-doped carbon encapsulated CoP hollow nanofibers as multifunctional electrode materials for potassium-ion and lithium-sulfur batteries.

Herein, a composite of N-doped carbon coated phosphating cobalt hollow nanofibers (N/C@CoP-HNFs) was synthesized by electrospinning, phosphating, and carbon coating processes. When employed as multifunctional electrode materials for potassium-ion batteries (PIBs) and lithium-sulfur (Li-S) batteries, the N/C@CoP-HNFs demonstrated notable electrochemical properties. Specifically, it delivered an initial specific capacity of 420.4 mA h g-1 at a current density of 100 mA g-1, with a sustained capacity of 190.8 mA h g-1 after 200 cycles in PIBs, and a specific capacity of 1448 mA h g-1 at a current density of 0.5C in Li-S batteries, which is considered relatively high for these types of battery technology. This good performance may due to the combination of the carbon nitrogen layer and cobalt phosphide bilayer hollow tube structure, which is conducive to telescoping the diffusion length of ions and electrons and buffer volume variation, and effectively inhibits the shuttle effect. Density functional theory (DFT) calculations were also used to explore the energy storage mechanism of the material. The possible adsorption sites and corresponding adsorption energy of K+ were analyzed, and the advantages of the material were explored by calculating the diffusion barrier and state density. The theoretical simulations further validated the strong adsorption capability of CoP for polysulfides. This work is expected to provide new ideas for new energy storage materials.

Electrochemical Failure Mechanism of δ-MnO2 in Zinc Ion Batteries Induced by Irreversible Layered to Spinel Phase Transition.

Phase transitions of Mn-based cathode materials associated with the charge and discharge process play a crucial role on the rate capability and cycle life of zinc ion batteries. Herein, a microscopic electrochemical failure mechanism of Zn-MnO2 batteries during the phase transitions from δ-MnO2 to λ-ZnMn2O4 is presented via systematic first-principle investigation. The initial insertion of Zn2+ intensifies the rearrangement of Mn. This is completed by the electrostatic repulsion and co-migration between guest and host ions, leading to the formation of λ-ZnMn2O4. The Mn relocation barrier for the λ-ZnMn2O4 formation path with 1.09 eV is significantly lower than the δ-MnO2 re-formation path with 2.14 eV, indicating the irreversibility of the layered-to-spinel transition. Together with the phase transition, the rearrangement of Mn elevates the Zn2+ migration barrier from 0.31 to 2.28 eV, resulting in poor rate performance. With the increase of charge-discharge cycles, irreversible and inactive λ-ZnMn2O4 products accumulate on the electrode, causing continuous capacity decay of the Zn-MnO2 battery.

Pure Amorphous and Ultrathin Phosphate Layer with Superior Ionic Conduction for Zinc Anode Protection.

Fast and uniform ion transport within the solid electrolyte interphase (SEI) is considered a crucial factor for ensuring the long-term stability of metal electrodes. In this study, we present the fabrication of ultrathin artificial interphases consisting of a zinc phosphate nanofilm with pure amorphous characteristics and a surfactant overlayer. The thickness of the interphases can be precisely controlled within the range of a few tens of nanometers. We explore the impact of artificial SEI structure, including thickness and crystallinity, on its protective capabilities. The pure amorphous phosphate layer with optimized nanoscale thickness is found to provide an abundance of short and isotropic ion migration pathways and a low diffusion energy barrier. These features facilitate rapid and homogeneous Zn2+ transportation, resulting in compact and planar zinc deposition. Meanwhile, the hydrophobic alkyl moieties of the overlayer prevent disassociation of water at the interface. As a result, this nanofilm endures ultralong cycling stability with a low overpotential and enables high Zn plating/stripping reversibility. The Zn||MnO2 full cell shows a stable cycle life for 700 cycles under practical conditions of lean electrolyte, high areal capacity cathode, and limited Zn excess. These findings provide insights into the design and optimization of SEI layers for protection of metal anodes.

Activating Selenium Cathode Chemistry for Aqueous Zinc-Ion Batteries.

Aqueous rechargeable zinc-ion batteries (ARZIBs) are a promising next-generation energy-storage device by virtue of the superior safety and low cost of both the aqueous electrolyte and zinc-metal anode. However, their development is hindered by the lack of suitable cathodes with high volumetric capacity that can provide both lightweight and compact size. Herein, a novel cathode chemistry based on amorphous Se doped with transition metal Ru that mitigates the resistive surface layer produced by the side reactions between the Se cathode and aqueous electrolyte is reported. This improvement can permit high volumetric capacity in this system. Distinct from the conventional conversion mechanisms between Se and ZnSe in Se||Zn cells, this strategy realizes synchronous proton and Zn2+ intercalation/deintercalation in the Ru-doped amorphous Se||Zn half cells. Moreover, an unanticipated Zn2+ deposition/stripping process in this system further contributes to the superior electrochemical performance of this new cathode chemistry. Consequently, the Ru-doped amorphous Se||Zn half cells are found to deliver a record-high capacity of 721 mAh g-1 /3472 mAh cm-3 , and superior cycling stability of over 800 cycles with only 0.015% capacity decay per cycle. This reported work opens the door for new chemistries that can further improve the gravimetric and volumetric capacity of ARZIBs.

Selective Cocatalyst Decoration of Narrow-Bandgap Broken-Gap Heterojunction for Directional Charge Transfer and High Photocatalytic Properties.

Narrow-bandgap semiconductors are promising photocatalysts facing the challenges of low photoredox potentials and high carrier recombination. Here, a broken-gap heterojunction Bi/Bi2 S3 /Bi/MnO2 /MnOx , composed of narrow-bandgap semiconductors, is selectively decorated by Bi, MnOx nanodots (NDs) to achieve robust photoredox ability. The Bi NDs insertion at the Bi2 S3 /MnO2 interface induces a vertical carrier migration to realize sufficient photoredox potentials to produce O2 •- and • OH active species. The surface decoration of Bi2 S3 /Bi/MnO2 by Bi and MnOx cocatalysts drives electrons and holes in opposite directions for optimal photogenerated charge separation. The selective cocatalysts decoration realizes synergistic surface and bulk phase carrier separation. Density functional theory (DFT) calculation suggests that Bi and MnOx NDs act as active sites enhancing the absorption and reactants activation. The decorated broken-gap heterojunction demonstrates excellent performance for full-light driving organic pollution degradation with great commercial application potential.

First-principles calculations of bulk WX2(X = Se, Te) as anode materials for Na ion battery.

Two-dimensional transition metal dichalcogenides are promising anode materials for Na ion batteries (NIBs). In this study, we carried out a comprehensive investigation to analyze the structural, electrochemical characteristics, and diffusion kinetics of bulk WX2(X = Se, Te) by employing first-principles calculation in the framework of density functional theory. We deeply studied the full intercalation of Na+in WX2and diagnosed NayX phase through conversion reaction mechanism. The voltage range of 2.05-0.48 V vs Na/Na+for NayWSe2and 2.26-0.65 V for NayWTe2(y= 0-3) have been noted. Density of states analysis showed metallic behavior of WX2(X = Se, Te) during sodiation. The facile pathways for Na+mobility through WX2have shown that tungsten dichalcogenides are inferred as excellent electrode material for NIBs.

Phonon and Exciton Properties between WS2 and MoS2 Layers via Inversion Heterostructure Engineering.

Recently, two-dimensional (2D) van der Waals heterostructures (vdWHs) have exhibited emergent electronic and optical properties due to their peculiar phonons and excitons, which lay the foundation for the development of photoelectronic devices. The dielectric environment plays an important role in the interlayer coupling of vdWHs. Here, we studied the interlayer and extra-layer dielectric effects on phonon and exciton properties in WS2/MoS2 and MoS2/WS2 vdWHs by Raman and photoluminescence (PL) spectroscopy. The ultralow frequency (ULF) Raman modes are insensitive to atomic arrangement at the interface between 1LW and 1LM and dielectric environments of neighboring materials, and the layer breathing mode (LBM) frequency follows that of WS2. The shift of high-frequency (HF) Raman modes is attributable to interlayer dielectric screening and charge transfer effects. Furthermore, the energy of interlayer coupling exciton peak I is insensitive to atomic arrangement at the interface between 1LW and 1LM and its energy follows that of MoS2, but the slight intensity difference in inversion vdWHs means that the substrate's dielectric properties may induce doping on the bottom layer. This paper provides fundamental understanding of phonon and exciton properties of such artificially formed vdWHs structures, which is important for new insights into manipulating the performances of potential devices.

Graphdiyne/Graphene/Graphdiyne Sandwiched Carbonaceous Anode for Potassium-Ion Batteries.

Graphdiyne (GDY) has been considered as an appealing anode candidate for K-ion storage since its triangular pore channel, alkyne-rich structure, and large interlayer spacing would endow it with abundant active sites and ideal diffusion paths for K-ions. Nevertheless, the low surface area and disordered structure of bulk GDY typically lead to unsatisfied K storage performance. Herein, we have designed a GDY/graphene/GDY (GDY/Gr/GDY) sandwiched architecture affording a high surface area and fine quality throughout a van der Waals epitaxy strategy. As tested in a half-cell configuration, the GDY/Gr/GDY electrode exhibits better capacity output, rate capability, and cyclic stability as compared to the bare GDY counterpart. In situ electrochemical impedance spectroscopy/Raman spectroscopy/transmission electron microscopy are further applied to probe the K-ion storage feature and disclose the favorable reversibility of GDY/Gr/GDY electrode during repeated potassiation/depotassiation. A full-cell device comprising a GDY/Gr/GDY anode and a potassium Prussian blue cathode enables a high cycling stability, demonstrative of the promising potential of the GDY/Gr/GDY anode for K-ion batteries.

Designing of Efficient Bifunctional ORR/OER Pt Single-Atom Catalysts Based on O-Terminated MXenes by First-Principles Calculations.

MXenes have been used as substrate materials for single-atom catalysts (SACs) due to their unique two-dimensional (2D) structure, high surface area, and high electronic conductivity. Oxygen is the primary terminating group of MXenes; however, all of the reported Pt SACs till now are fabricated with F-terminated MXenes. According to the first-principles calculations of this work, the failure of using O-terminated MXenes as substrates is due to the low charge density around Pt and C, which weakens the catalytic activity of Pt. By adjusting the electronic structure of M2C using a second submetal with a lower work function than M, 18 potential bifunctional Pt SACs are constructed based on O-terminated bimetal MXenes. After further consideration of some important practical application factors such as overpotential, solvation effect, and reaction barriers, only four of them, i.e., Cr2Nb2C3O2-VO-Pt, Cr2Ta2C3O2-VO-Pt, Cr2NbC2O2-VO-Pt, and Cr2TaC2O2-VO-Pt, are screened as bifunctional oxygen reduction reaction/oxygen evolution reaction (ORR/OER) catalysts. All of these screened SACs are originated from Cr-based MXenes, implying the significance of Cr-based MXenes in designing bifunctional Pt SACs.

Magnesium Ion Storage Properties in a Layered (NH4)2V6O16·1.5H2O Nanobelt Cathode Material Activated by Lattice Water.

Magnesium ion batteries have attracted increasing attention as a promising energy storage device due to the high safety, high volumetric capacity, and low cost of Mg. However, the strong Coulombic interactions between Mg2+ ions and cathode materials seriously hinder the electrochemical performance of the batteries. To seek a promising cathode material for magnesium ion batteries, in this work, (NH4)2V6O16·1.5H2O and water-free (NH4)2V6O16 materials are synthesized by a one-step hydrothermal method. The effects of NH4+ and lattice water on the Mg2+ storage properties in these kinds of layered cathode materials are investigated by experiments and first-principles calculations. Lattice water is demonstrated to be of vital importance for Mg2+ storage, which not only stabilizes the layered structure of (NH4)2V6O16·1.5H2O but also promotes the transport kinetics of Mg2+. Electrochemical experiments of (NH4)2V6O16·1.5H2O show a specific capacity of 100 mA·h·g-1 with an average discharge voltage of 2.16 V vs Mg2+/Mg, highlighting the potential of (NH4)2V6O16·1.5H2O as a high-voltage cathode material for magnesium ion batteries.

Strain-driven phase transition and spin polarization of Re-doped transition-metal dichalcogenides.

Two-dimensional transition metal dichalcogenides (TMDCs) are promising in spintronics due to their spin-orbit coupling, but their intrinsic non-magnetic properties limit their further development. Here, we focus on the energy landscapes of TMDC (MX2, M = Mo, W and X = S, Se, Te) monolayers by rhenium (Re) substitution doping under axial strains, which controllably drive 1H ↔ 1Td structural transformations. For both 1H and 1Td phases without strain, Re-doped TMDCs have an n-type character and are non-magnetic, but the tensile strain could effectively induce and modulate the magnetism. Specifically, 1H-Re0.5Mo0.5S2 gets a maximum magnetic moment of 0.69 µB at a 6% uniaxial tensile strain along the armchair direction; along the zigzag direction it exhibits a significant magnetic moment (0.49 µB) at a 2.04% uniaxial tensile strain but then exhibits no magnetism in the range of [5.10%, 7.14%]. By contrast, for 1Td-Re0.5Mo0.5S2 a critical uniaxial tensile strain along the zigzag direction reaches up to ∼9.18%, and a smaller uniaxial tensile strain (∼5.10%) along the zigzag direction is needed to induce the magnetism in 1Td-Re0.5M0.5Te2. The results reveal that the magnetism of Re-doped TMDCs could be effectively induced and modulated by the tensile strain, suggesting that strain engineering could have significant applications in doped TMDCs.

Mechanisms of sodiation in anatase TiO2 in terms of equilibrium thermodynamics and kinetics.

Anatase TiO2 is a promising anode material for sodium-ion batteries (SIBs). However, its sodium storage mechanisms in terms of crystal structure transformation during sodiation/de-sodiation processes are far from clear. Here, by analyzing the redox thermodynamics and kinetics under near-equilibrium states, we observe, for the first time, that upon Na-ion uptake, the anatase TiO2 undergoes a phase transition and then an irreversible crystal structure disintegration. Additionally, unlike previous theoretical studies which investigate only the two end points of the sodiation process (i.e., TiO2 and NaTiO2), we study the progressive crystal structure changes of anatase TiO2 upon step-by-step Na-ion uptake (Na x TiO2, x = 0.0625, 0.125, 0.25, 0.5, 0.75, and 1) for the first time. It is found that the anatase TiO2 goes through a thermodynamically unstable intermediate phase (Na0.25TiO2) before reaching crystalline NaTiO2, confirming the inevitable crystal structure disintegration during sodiation. These combined experimental and theoretical studies provide new insights into the sodium storage mechanisms of TiO2 and are expected to provide useful information for further improving the performance of TiO2-based anodes for SIB applications.

Enhanced electro-Fenton degradation of sulfonamides using the N, S co-doped cathode: Mechanism for H2O2 formation and pollutants decay.

Facing low reactivity/selectivity of oxygen reduction reaction (ORR) in electro-Fenton (EF), N, S atoms were introduced into carbon-based cathode. "End-on" O2 adsorption was achieved by adjusting electronic nature via N doping, while *OOH binding capability was tuned by spin density variation via S doping. Results showed the optimized N, S co-doped cathode presented a 42.47% improvement of H2O2 accumulation (7.95 ± 0.02 mg L-1 cm-2). According to density functional theory (DFT), N, S co-doped structure favored the "end-on" O2 adsorption as adsorption energy dropped to - 2.24 eV. Moreover, O-O/C-O bond lengths variation proved a possibility for *OOH desorption. The elaborated cathode was used in EF for sulfonamides (SAs) decay. A 100% removal rate of sulfadiazine (SDZ), sulfathiazole (STZ) and sulfadimethoxine (SDM) was achieved within 60 min, among which SDZ tended to be degraded easily. Because the absolute hardness (η) of those pollutants is ranked as follows: ηSDM> Î·STZ> Î·SDZ. Degradation pathways were proposed based on the detected byproducts, along with toxicity was evaluated by ecological structure-activity relationship (ECOSAR) program. Results showed that toxic intermediates generated were reduced or even disappeared. EF with N, S co-doped cathode provides a promising process for antibiotics wastewater treatment.

Three-Phase Boundary in Cross-Coupled Micro-Mesoporous Networks Enabling 3D-Printed and Ionogel-Based Quasi-Solid-State Micro-Supercapacitors.

The construction of advanced micro-supercapacitors (MSCs) with both wide working-voltage and high energy density is promising but still challenging. In this work, a series of nitrogen-doped, cross-coupled micro-mesoporous carbon-metal networks (N-STC/Mx Oy ) is developed as robust additives to 3D printing inks for MSCs fabrication. Taking the N-STC/Fe2 O3 nanocomposite as an example, both experimental results and theoretical simulations reveal that the well-developed hierarchical networks with abundantly decorated ultrafine Fe2 O3 nanoparticles not only significantly facilitate the ion adsorption at its three-phase boundaries (Fe2 O3 , N-STC, and electrolyte), but also greatly favor ionic diffusion/transport with shortened pathways. Consequently, the as-prepared N-STC/Fe2 O3 electrode delivers a high gravimetric capacitance (267 F g-1 at 2 mV s-1 ) and outstanding stability in a liquid-electrolyte-based symmetric device, as well as a record-high energy density of 114 Wh kg-1 for an asymmetric supercapacitor. Particularly, the gravimetric capacitance of the ionogel-based quasi-solid-state MSCs by 3D printing reaches 377 F g-1 and the device can operate under a wide temperature range (-10 to 60 °C).

Induction of Planar Sodium Growth on MXene (Ti3C2Tx)-Modified Carbon Cloth Hosts for Flexible Sodium Metal Anodes.

Sodium (Na) metal batteries have attracted increasing attention and gained rapid development. However, the processing, storing, and application of Na metal anodes are restricted by its inherent stickiness and poor mechanical properties. Herein, an MXene (Ti3C2Tx)-coated carbon cloth (Ti3C2Tx-CC) host is designed and synthesized, which shows a highly metallic conductive and sodiophilic surface. After a thermal infusion treatment, a Na-Ti3C2Tx-CC composite with rigidity and bendability is obtained and employed as a metal anode for Na ion batteries. The Na-Ti3C2Tx-CC electrodes present stable cycling performance and high stripping/plating capacity in both an ether-based (up to 5 mA·h·cm-2) and a carbonate-based (up to 8 mA·h·cm-2) electrolyte. The fundamental protection mechanism of MXene Ti3C2Tx is investigated. Ti3C2Tx efficiently induces Na's initial nucleation and laterally oriented deposition, which effectively avoids the generation of mossy/dendritic Na. The arrangement of Na atoms deposited on the MXene surface inherits the MXene atomic architecture, leading to a smooth "sheet-like" Na surface. Meanwhile, a flexible Na-based Na-Ti3C2Tx-CC∥Na3V2(PO4)3 device is assembled and exhibits capable electrochemical performance.

Experimental Investigation and First-Principles Calculations of a Ni3Se4 Cathode Material for Mg-Ion Batteries.

Magnesium ion batteries (MIBs) have attracted increasing attention due to their advantages of abundant reserves, low price, and high volumetric capacity. However, the large Coulombic interactions of Mg2+ with the cathode framework seriously hinder the rate capability and cycle stability of the battery cell. For this reason, finding a suitable cathode material has become a main task in MIB research. In this study, Ni3Se4 was first proposed as a new cathode material for MIBs. First-principles calculations showed that Ni3Se4 could accommodate up to 1 mol of Mg2+, but the migration energy barrier was as high as 1.35 eV. Accordingly, nanosized Ni3Se4 was prepared by a hydrothermal method to achieve satisfying electrochemical performance. The prepared Ni3Se4 material showed a discharge capacity of 99.8 mA·h·g-1 at 50 mA·g-1 current density with a capacity retention of 75% after 100 cycles. Combined with first-principles calculations and spectroscopic studies, it was demonstrated that the material underwent a solid-solution structural change during Mg2+ insertion, with all charge transfer taking place on the Ni cations.

Q-Carbon: A New Carbon Allotrope with a Low Degree of s-p Orbital Hybridization and Its Nucleation Lithiation Process in Lithium-Ion Batteries.

A novel metallic carbon allotrope, Q-carbon, was discovered using first-principles calculations. The named Q-carbon possessed a three-dimensional (3D) cage structure formed by carbon atoms with three ligands. The energy distribution of electrons in different orbitals revealed that Q-carbon has a low degree of s-p orbital hybridization. The calculated Li+ binding energies suggested Li+ aggregation inside Q-carbon during lithiation. As a result, a Li8C32 phase was formed and gradually expanded in Q-carbon, implying a typical two-phase transition. This allowed Q-carbon to have a constant theoretical voltage of 0.40 V, which effectively inhibited Li dendrite formation. A stable Li8C32/C32 two-phase interface was confirmed by stress-strain analysis, and a calculated Li+ diffusion barrier of ∼0.50 eV ensured effective Li+ diffusion along a 3D pathway. This study was of great significance for the understanding of two-phase transition of Li+ storage materials and provided a new insight into the design of new carbon materials for energy storage applications.

Lithiophilic Three-Dimensional Porous Ti3C2Tx-rGO Membrane as a Stable Scaffold for Safe Alkali Metal (Li or Na) Anodes.

Metallic anodes have high theoretical specific capacities and low electrochemical potentials. However, short-circuit problems caused by dendritic deposition and low Coulombic efficiency limit the cyclic life and safety of metallic anode-based batteries. Herein, dendrite-free and flexible three-dimensional (3D) alkali anodes (Li/Na-Ti3C2Tx-rGO) are constructed by infusing molten lithium (Li) or sodium (Na) metal into 3D porous MXene Ti3C2Tx-reduced graphene oxide (Ti3C2Tx-rGO) membranes. First-principles calculations indicate that large fractions of functional groups on the Ti3C2Tx surface lead to the good affinity between the Ti3C2Tx-rGO membrane and molten alkali metal (Li/Na), and the formation of Ti-Li/Na, O-Li/Na, and F-Li/Na mixed covalent/ionic bonds is extremely critical for uniform electrochemical deposition. Furthermore, the porous structure in Li/Na-Ti3C2Tx-rGO composites results in an effective encapsulation, preventing dendritic growth and exhibiting stable stripping/plating behaviors up to 12 mA·cm-2 and a deeper capacity of 10 mA·h·cm-2. Stable cycling performances over 300 h (750 cycles) at 5.0 mA·cm-2 for Li-Ti3C2Tx-rGO and 500 h (750 cycles) at 3.0 mA·cm-2 for Na-Ti3C2Tx-GO are achieved. In a full cell with LiFePO4 cathodes, Li-Ti3C2Tx-rGO electrodes show low polarization and retain 96.6% capacity after 1000 cycles. These findings are based on 2D MXene materials, and the resulting 3D host provides a practical approach for achieving stable and safe alkali metal anodes.

A General Atomic Surface Modification Strategy for Improving Anchoring and Electrocatalysis Behavior of Ti3C2T2 MXene in Lithium-Sulfur Batteries.

Multiple negative factors, including the poor electronic conductivity of sulfur, dissolution and shuttling of lithium polysulfides (Li2Sn), and sluggish decomposition of solid Li2S, seriously hinder practical applications of lithium-sulfur (Li-S) batteries. To solve these problems, a general strategy was proposed for enhancing the electrochemical performance of Li-S batteries using surface-functionalized Ti3C2 MXenes. Functionalized Ti3C2T2 (T = N, O, F, S, and Cl) showed metallic conductivity, as bare Ti3C2. Among all Ti3C2T2 investigated, Ti3C2S2, Ti3C2O2, and Ti3C2N2 offered moderate adsorption strength, which effectively suppressed Li2Sn dissolution and shuttling. This Ti3C2T2 exhibited effective electrocatalytic ability for Li2S decomposition. The Li2S decomposition barrier was significantly decreased from 3.390 eV to ∼0.4 eV using Ti3C2S2 and Ti3C2O2, with fast Li+ diffusivity. Based on these results, O- and S-terminated Ti3C2 were suggested as promising host materials for S cathodes. In addition, appropriate functional group vacancies could further promote anchoring and catalytic abilities of Ti3C2T2 to boost the electrochemical performance of Li-S batteries. Moreover, the advantages of a Ti3C2T2 host material could be well retained even at high S loading, suggesting the potential of surface-modified MXene for confining sulfur in Li-S battery cathodes.