Charge Dynamics Engineering Sparks Hetero-Interfacial Polarization for an Ultra-Efficient Microwave Absorber with Mechanical Robustness.

Microwave absorbers with high efficiency and mechanical robustness are urgently desired to cope with more complex and harsh application scenarios. However, manipulating the trade-off between microwave absorption performance and mechanical properties is seldom realized in microwave absorbers. Here, a chemistry-tailored charge dynamic engineering strategy is proposed for sparking hetero-interfacial polarization and thus coordinating microwave attenuation ability with the interfacial bonding, endowing polymer-based composites with microwave absorption efficiency and mechanical toughness. The absorber designed by this new conceptual approach exhibits remarkable Ku-band microwave absorption efficiency (-55.3 dB at a thickness of 1.5 mm) and satisfactory effective absorption bandwidth (5.0 GHz) as well as desirable interfacial shear strength (97.5 MPa). The calculated differential charge density depicts the uneven distribution of space charge and the intense hetero-interfacial polarization, clarifying the structure-performance relationship from a theoretical perspective. This work breaks through traditional single performance-oriented design methods and ushers a new direction for next-generation microwave absorbers.

Interlayer Sodium Plating/Stripping in Van der Waals-Layered Quantum Dot Superstructure.

Assembling quantum dots (QDs) into van der Waals (vdW)-layered superstructure holds great promise for the development of high-energy-density metal anode. However, designing such a superstructure remains to be challenging. Here, a chemical-vapor Oriented Attachment (OA) growth strategy is proposed to achieve the synthesis of vdW-layered carbon/QDs hybrid superlattice nanosheets (Fe7 S8 @CNS) with a large vdW gap of 3 nm. The Fe7 S8 @CNS superstructure is assembled by carbon-coated Fe7 S8 (Fe7 S8 @C) QDs as building blocks. Interestingly, the Fe7 S8 @CNS exhibits two kinds of edge dislocations similar to traditional atom-layered materials, suggesting that Fe7 S8 @C QDs exhibit quasi-atomic growth behavior during the OA process. More interestingly, when used as host materials for sodium metal anodes, the Fe7 S8 @CNS shows the interlayer sodium plating/stripping behavior, which well suppresses Na dendrite growth. As a result, the cell with Fe7 S8 @CNS anode can keep stable cycling for 1000 h with a high Coulombic efficiency (CE) of ≈99.5% at 3.0 mA cm-2 and 3.0 mAh cm-2 . Noticeably, the Na@Fe7 S8 @CNS||Na3 V2 (PO4 )3 full cells can attain a capacity of 88.8 mAh g-1 with a retention of 97% after 1000 cycles at 1.0 A g-1 (≈8 C), showing excellent cycle stability for practical applications. This work enriches the vdW-layered QDs superstructure family and their application toward energy storage.

Borderline Metal Centers on Nonporous Metal-Organic Framework Nanowire Boost Fast Li-Ion Interfacial Transport of Composite Polymer Electrolyte.

Metal-organic frameworks (MOFs) fillers are emerging for composite polymer electrolytes (CPEs). Enhancing Lewis acid-base interaction (LABI) among MOFs, polymer and Li-salt is expected to promote Li+ -transport. However, it is unclear how to customize a strong LABI interface. The large surface-area of classical MOFs also interferes with clarifying the LABI influence on Li+ -transport. Herein, Bi3+ as metal centers to design colloidal-dispersed nonporous MOFs (Bi/HMT-MOFs) nanowire with a surface-area of only 17.13 m2 g-1 to prepare polyethylene oxide (PEO)-based CPEs (BMCPE) is chosen. The nonporous feature can exclude the surface-area effect on Li+ -transport. More interestingly, Bi3+ is a typical borderline acid, which can interact with both hard-basic PEO and soft-basic Li-salt anion. Accordingly, Bi/HMT-MOFs are uniformly dispersed in the BMCPE to form a strong LABI interface with PEO and Li-salt, promoting Li-salt dissociation and providing rapid Li+ -transport channels. Despite the ultralow surface-area of Bi/HMT-MOFs, BMCPE exhibits significantly enhanced ion-conductivity and Li+ transference number, which completely rival traditional MOFs-filled CPEs. BMCPE also enables symmetric and full cells with excellent high-rate performance and long-term cycling stability. In contrast, when Bi3+ sites are obscured, electrochemical performances are obviously decreased. Therefore, employing borderline metal centers will be an effective strategy to construct a LABI interface for high-performance MOFs-filled CPEs.

Integrating Multi-Heterointerfaces in a 1D@2D@1D Hierarchical Structure via Autocatalytic Pyrolysis for Ultra-Efficient Microwave Absorption Performance.

Developing microwave absorption (MA) materials with ultrahigh efficiency and facile preparation method remains a challenge. Herein, a superior 1D@2D@1D hierarchical structure integrated with multi-heterointerfaces via self-assembly and an autocatalytic pyrolysis is designed to fully unlock the microwave attenuation potential of materials, realizing ultra-efficient MA performance. By precisely regulating the morphology of the metal organic framework precursor toward improved impedance matching and intelligently integrating multi-heterointerfaces to boosted dielectric polarization, the specific return loss value of composites can be effectively tuned and optimized to -1002 dB at a very thin thickness of 1.8 mm. These encouraging achievements shed fresh insights into the precise design of ultra-efficient MA materials.

Antiblocking Heterostructure to Accelerate Kinetic Process for Na-Ion Storage.

Heterostructures are attracting increasing attention in the field of sodium-ion batteries. However, it is still unclear whether any two monophase components can be used to construct a high-performance heterostructure for sodium-ion batteries, as well as the kind of heterostructures that can boost electrochemical performances. In this study, based on classical semiconductor theories on antiblocking and blocking interfaces, attempts are made to answer the abovementioned queries. For this purpose, NiTe2 -ZnTe antiblocking and CoTe2 -ZnTe blocking heterostructures are synthesized through a bimetal-hexamine framework-derived strategy. The NiTe2 -ZnTe antiblocking heterostructure exhibits excellent high-rate and cycling performances, while the CoTe2 -ZnTe blocking heterostructure performs poorly, even compared to their monophase components. Further, kinetic measurements and theoretical calculation confirm that antiblocking heterointerfaces can boost Na-ion diffusion efficiency and decrease the diffusion barrier, which can be attributed to the highly conductive antiblocking heterointerfaces generated due to electron transfer from NiTe2 to ZnTe. Therefore, this study provides a new perspective to design heterostructures more efficiently, with significantly better Na-ion storage performance.

Tailoring Highly N-Doped Carbon Materials from Hexamine-Based MOFs: Superior Performance and New Insight into the Roles of N Configurations in Na-Ion Storage.

To prepare highly N-doped carbon materials (HNCs) as well as to determine the influence of N dopants on Na-ion storage performance, hexamine-based metal-organic frameworks are employed as new and efficient precursors in the preparation of HNCs. The HNCs possess reversible capacities as high as 160 and 142 mA h g-1 at 2 A g-1 (≈8 C) and 5 A g-1 (≈20 C), respectively, and maintain values of 145 and 123 mA h g-1 after 500 cycles, thus exhibiting excellent rate and long-term cyclic performance. Based on systematic analysis, a new insight into the roles of the different N configurations in Na-ion storage is proposed. The adsorption of Na ions on pyridinic-N (N-6) and pyrrolic-N (N-5) is fully irreversible, whereas the adsorption on graphitic-N (N-Q) is partially reversible and the adsorption on N-oxide (N-O) is fully reversible. More importantly, the N-6/N-Q ratio is an intrinsic parameter that reflects the relationship between the N configurations and carbon textures for N-doped carbons prepared from in situ pyrolysis of organic precursors. The cyclic stability and rate-performance improve with decreasing N-6/N-Q ratio. Therefore, this work is of great significance for the design of N-doped carbon electrodes with high performance for sodium ion batteries.

Phenolic resin-grafted reduced graphene oxide as a highly stable anode material for lithium ion batteries.

A novel and effective route for preparing phenol formaldehyde resin grafted reduced graphene oxide (rGO-g-PF) electrode materials with highly enhanced electrochemical properties is reported. In order to prepare rGO-g-PF, hydroxymethyl-terminated PF is initially grafted to graphene oxide (GO) via esterification reaction. Subsequently, the grafted GO is reduced by the carbonization process under an inert gas atmosphere. The covalent linkage, morphology, thermal stability and electrochemical properties of rGO-g-PF are systematically investigated by Fourier transform infrared spectroscopy, scanning electron microscopy, thermal gravimetric analysis, differential scanning calorimetry and a variety of electrochemical testing techniques. In the constructed architecture, the amorphous carbon shell can inhibit the co-intercalation of solvated lithium ion and avoid partial exfoliation of the graphene layers, thus effectively reducing the irreversible capacity and preserving the structural integrity. Meanwhile, the carbon coating layer leading to a decreased thickness of SEI film can improve the conductivity of electrode materials. As a result, the rGO-g-PF electrode exhibits impressive high cycling stability at various large current densities (376.5 mA h g(-1) at 50 mA g(-1) for 250 cycles, 337.8 mA h g(-1) at 200 mA g(-1) and 267.8 mA h g(-1) at 1 A g(-1) for 200 cycles), in combination with high rate capability.

Surfactant-assisted molecular-level tunning of phenol-formaldehyde-based hard carbon microspheres for high-performance sodium-ion batteries.

The phenol-formaldehyde (PF) resin is an economical precursor for spherical hard carbon (HC) anodes for sodium-ion batteries (SIBs). However, achieving precise molecular-level control of PF-based HC microspheres, particularly for optimizing ion transport microstructure, is challenging. Here, a sodium linoleate (SL)-assisted strategy is proposed to enable molecular-level engineering of PF-based HC microspheres. PF microspheres are synthesized through the polymerization of 3-aminophenol and formaldehyde, initially forming oxazine rings and then undergoing ring-opening polymerization to create a macromolecular network. SL functions as both a surfactant to control microsphere size and a catalyst to enhance ring-opening polymerization and increase polymerization of PF resin. These modifications lead to reduced microsphere diameter, increased interlayer spacing, enhanced graphitization, and significantly improved electron and ion transfer. The synthesized HC microspheres exhibit a remarkable reversible capacity of 337 mAh/g, maintaining 96.9 mAh/g even at a high current density of 5.0 A/g. Furthermore, the full cell demonstrates a high capacity of 150 mAh/g, an energy density of 125.3 Wh kg-1, an impressive initial coulombic efficiency (ICE) of 930.3% at 1 A/g, and remarkable long-term stability over 3000 cycles. This study highlights the potential of surfactant-assisted molecular-level engineering in customizing HC microspheres for advanced SIBs.

Carbon-Confined Two-Dimensional Sodiophilic Sites Boosted Dendrite-Free Sodium Metal Anodes.

Carbon-supported sodium metal anodes (SMAs) have attracted growing interest in next-generation energy storage applications. Sodiophilic sites on carbon hosts such as foreign metal/metal compounds are critical for suppressing Na dendrite growth. However, the foreign active materials are mostly restricted to nanoparticle-like structures, which suffer from severe agglomeration and low metal utilization. Here, we develop the carbon-encapsulated mosaic Fe3O4 nanosheets (Fe3O4@CNS) with two-dimensional (2D) active sites via the oriented attachment (OA) mechanism. Ultrathin Fe3O4 nanosheets not only endow the carbon hosts with a continuous 2D nucleation region and high metal utilization but also catalyze the formation of a stable solid electrolyte interphase (SEI) film. Additionally, carbon shells can protect the Fe3O4 against electrolyte exfoliation. As a result, the Fe3O4@CNS half cells achieve a cycle of up to 1800 h with an average Coulombic efficiency (CE) of 99.6% at 1.0 mA cm-2 and 1.0 mA h cm-2 and still stably cycle for 800 h with a high CE of 99.2% even at 3.0 mA cm-2 and 3.0 mA h cm-2. The Na@Fe3O4@CNS symmetric cells can last for more than 2200 h at 1.0 mA cm-2 and 1.0 mA h cm-2. And the Na3V2(PO4)3 || Na@Fe3O4@CNS full cells can attain a specific capacity of 86.6 mA h g-1 after 350 cycles at 1.0 A g-1 (∼8C), showing excellent cycle stability for practical applications. This work provides a new method to establish efficient 2D nucleation sites in the Na hosts.

Na Storage Activity or Inertness of P-Configurations in N, P Dual-Doped Carbon Nanofibers: Bulk vs Surface.

Heteroatom doping is an effective method to improve the electrochemical properties of hard carbon anodes for sodium-ion batteries. However, the different roles of surface and bulk heteroatoms in Na storage have not been explored much. Herein, N, P dual-doped carbon nanofibers (NP-CNFs) with high doping contents and low surface area are designed to clarify the above issue. It is confirmed that P plays a more crucial role in Na storage compared with N. In addition, surface and bulk P not only possess different configurations but also show distinct Na storage activity. There are only oxidized POx groups on the surface, which are inactive for Na storage but promote the stability of electrochemistry interphase, while in the bulk phase, unoxidized P-C bonds also emerge except POx, which shows preeminently reversible Na storage activity, and the POx groups are activated simultaneously. Furthermore, P-doping changes the reactivity of N-configurations with Na both on the surface and in the bulk phase, exhibiting interesting synergism. As expected, the surface stability, bulk activity, and synergism enable NP-CNFs to achieve superior performance. It could deliver a prominent capacity of 105.6 mAh g-1 at 10 A g-1 after 3000 cycles in half cells and 164.3 mAh g-1 at 1 A g-1 after 200 cycles in full cells.

Lithiophilic onion-like carbon spheres as lithium metal uniform deposition host.

Lithium metal is considered as a promising anode material for next-generation secondary batteries, owing to its high theoretical specific capacity (3860 mA h g-1). Nevertheless, the practical application of Li in lithium metal batteries (LMBs) is hampered by inhomogeneous Li deposition and irreversible "dead Li", which lead to low coulombic efficiency (CE) and safe hazards. Designing unique lithiophilic structure is an efficient strategy to control Li uniformly plating /stripping. Here, we report the silver (Ag) nanoparticles coated with nitrogen-doped onion-like carbon microspheres (Ag@NCS) as a host to reduce the nucleation overpotential of Li for dendrite-free LBMs. The Ag@NCS were prepared by a simple one-step injection pyrolysis. The lithiophilic Ag is demonstrated to be priority selective deposition of Li in the carbon cage. Meanwhile, the onion-like structure benefits to uniform lithium nucleation and dendrite-free lithium during cycling. Impressively, we successfully captured lithium metal on different hosts at atomic scale, further proving that Ag@NCS can effectively and uniformly deposit Li. Besides, Ag@NCS show a superiorly electrochemical performance with a low nucleation overpotential (∼1 mV), high CE and stable cycling performance (over 400 cycles at 0.5 mA cm-2) compared to the Ag-free onion-like carbon in LMBs. Even under harsh conditions (1 mA cm-2, 4 mA h cm-2), Ag@NCS still present superior cycling stability for more than 150 cycles. Furthermore, a full cell composed of LiFePO4 cathode exhibits significantly improved voltage hysteresis with low voltage polarization. This work provides a new choice and route for the design and preparation of lithiophilic host materials.

Binary self-assembly of ordered Bi4Se3/Bi2O2Se lamellar architecture embedded into CNTs@Graphene as a binder-free electrode for superb Na-Ion storage.

With the development of various flexible electronic devices, flexible energy storage devices have attracted more research attention. Binder-free flexible batteries, without a current collector, binder, and conductive agent, have higher energy density and lower manufacturing costs than traditional sodium-ion batteries (SIBs). However, preparing binder-free anodes with high electrochemical performance and flexibility remains a great challenge. In this study, a binary self-assembly composite of an ordered Bi4Se3/Bi2O2Se lamellar architecture wrapped by carbon nanotubes (CNTs) was embedded in graphene with strong interfacial interaction to form Bi2O2Se/Bi4Se3@CNTs@rGO (BCG), which was used as a binder-free anode for SIBs. A unique "one-changes-into-two" phenomenon was observed: the layered Bi2Se3 was transformed into a unique layered Bi4Se3/Bi2O2Se heterojunction structure, which not only provides more electrochemical channels but also reduces internal stress to improve the stability of the material structure. BCG-2 showed excellent sodium-ion storage, delivering a reversible capacity of 346 mA h/g at 100 mA/g and maintaining a capacity of 235 mA h/g over 50 cycles. Even at a high current density of 1 A/g, it retains a capacity of 105 mA h/g after 1000 cycles. This unique design concept can also be employed in synthesizing other binder-free electrodes to improve their properties.

Diffusion of metal in a confined nanospace of carbon nanotubes induced by air oxidation.

Air oxidation can result in the motion of metal confined in carbon nanotubes (CNTs). This can also be utilized to tailor various hybrid nanostructures. By controllable air-oxidation, as-prepared metal@CNT nanorods (a) can be converted first to core-shell-void nanorods (b), then to metal/metal oxide@CNT nanotubes (c), and finally to mesoporous metal oxide nanotubes (d). The metal/metal oxide@CNT nanotubes and mesoporous metal oxide nanotubes are expected to find many applications, such as in lithium ion batteries, catalysis, magnetic drug delivery, and gas sensing.

Achieving Slope-Reigned Na-Ion Storage in Carbon Nanofibers by Constructing Defect-Rich Texture by a Cu-Activation Strategy.

Hard carbons have shown promising application potential as anode materials for sodium-ion batteries (SIBs), but adjusting the texture of hard carbons to manipulate their electrochemical behaviors remains a great challenge. In this work, a Cu-activation strategy is developed to control the defects of hard carbon nanofibers to achieve slope-reigned Na-ion storage behaviors. This method can effectively create defect-rich carbon texture by employing a small amount of Cu(NO3)2 as an activator but cannot induce an increase in the surface area. With the addition of the Cu activator, carbon nanofibers with increasing defects are synthesized by electrospinning and subsequent annealing. When carbon nanofibers are used as anodes for SIBs, their reversible capacity is increased with the increase of defects. Simultaneously, slope capacity gradually increases, while low-voltage plateau capacity reduces. Especially, the reversible capacity of Cu-activated nanofibers with more defects can be increased to 315 mA h g-1 with almost no plateau capacity compared with 203 mA h g-1 of inactivated nanofibers with a plateau capacity of 26%. Noticeably, the initial Coulombic efficiency (70%) of the activated nanofibers is just slightly lower than that (72%) of inactivated ones. The Cu-activated nanofibers also demonstrate superb rate performance and long cycle lifetime. Therefore, this work shows a new pathway for the design of defect-rich hard carbons with superior Na-ion storage performance.

Thermal-exfoliated synthesis of N-rich carbon-based nanosheets from layered bulk crystals of a metal-hexamine framework.

We develop a general strategy for the design of a series of 2D N-rich carbon-based nanomaterials through the thermal exfoliation of layered metal-hexamine framework microcrystals. By a facile pyrolysis, the sharply generated gases escape from the layered precursors, leading to the exfoliation of the layers and successful preparation of 2D carbon-based nanomaterials. This strategy can readily skip the complicated morphology engineering process of 2D metal-organic frameworks to produce 2D N-rich carbon-based nanomaterials on a large scale.

Graphene-Loaded Bi2Se3: A Conversion-Alloying-type Anode Material for Ultrafast Gravimetric and Volumetric Na Storage.

Sodium-ion battery (SIB) has been a promising alternative for sustainable electrochemical energy-storage devices. However, it still needs great efforts to develop electrode materials with ultrafast gravimetric and volumetric Na-storage performance, due to difficult balance between Na-ion diffusion kinetics and pressing density of materials. In this work, Bi2Se3/graphene composites, synthesized by a selenization reaction, are investigated as anode materials for SIBs. Na-ion storage mechanism of Bi2Se3 should be attributed to a combined conversion-alloying one by a series of ex situ measurements. In the composites, Bi2Se3 particles with an average diameter of 100 nm are uniformly dispersed onto graphene with strong interfacial interaction. Despite their nanoscale size, the pressing density of Bi2Se3/graphene composite could still reach a high value of 2.07 g/cm3. Therefore, the composites can deliver a high gravimetric specific capacity of 346 mAh/g and volumetric specific capacity of 716 mAh/cm3 at a current density of 0.1 A/g. Remarkably, the composites exhibit an ultrafast Na-storage capability and a negligible capacity fading with the increasing of current density from 0.2 to 5 A/g. Even at 10 A/g (≈30 C), the composites still possess a gravimetric capacity of 183 mAh/g and volumetric capacity of 379 mAh/cm3 with ultrastable cyclability up to 1000 cycles. This work introduces a valid route to design electrode materials with both excellent gravimetric and volumetric performance of Na-ion storage.

Achieving Ultrafast and Stable Na-Ion Storage in FeSe2 Nanorods/Graphene Anodes by Controlling the Surface Oxide.

Designing transitional metal selenides (TMSes) with superior rate and cyclic performance for sodium-ion storage remains great challenges. To achieve this task, the influence of surface oxides on Na-ion storage behavior of FeSe2 is investigated by designing FeSe2 with varying oxide content. It is found that surface oxide has an inhibitory effect on the activity of FeSe2. Small-sized FeSe2 on graphene with higher surface oxide content exhibits obviously inferior performance compared to large-sized FeSe2 with lower oxide content. By controlling oxide content, the prepared FeSe2 nanorods/graphene exhibits a high capacity of 459 mAh/g at 0.1 A/g and superior rate performance. Only 10% capacity decrease occurs with the increase in current density from 0.1 to 5 A/g. Even at 25 A/g (∼50 C), it delivers a capacity of 227 mAh/g with almost no decay after 800 cycles. The influence mechanism of surface oxide is investigated. The oxide can be converted to a sodiated shell with high mechanical strength and poor conductivity, which generates phase-transition resistance to suppress the sodiation of FeSe2 core, blocks the transfer of Na-ions and electrons in subsequent sodiation processes. Understanding the effect of surface oxide on Na-ion storage will be helpful in designing TMSes and other active materials.

Two-Dimensional NiSe2/N-Rich Carbon Nanocomposites Derived from Ni-Hexamine Frameworks for Superb Na-Ion Storage.

Design of functional carbon-based nanomaterials from metal-organic frameworks (MOFs) has attracted soaring interests in recent years. However, a MOF-derived strategy toward two-dimensional (2D) nanomaterials remains a great challenge. In this work, we develop a layered Ni-hexamine framework as efficient precursor to prepare a 2D NiSe2/N-rich carbon nanocomposite by a simple pyrolysis and subsequent selenization process. In the 2D NiSe2/N-rich carbon nanocomposite, NiSe2 nanoparticles with diameters of ca. 75 nm are homogeneously distributed in the N-rich carbon nanosheets. When serving as anode materials for sodium-ion batteries, the 2D nanocomposites exhibit a high reversible capacity of 410 mAh g-1 at 1 A g-1 and maintain a value of 255 mAh g-1 even at 10 A g-1. The excellent electrochemical performance can be attributed to the synergistic effects between the N-rich carbon nanosheets and NiSe2 nanoparticles. More importantly, the hexamine-based MOFs can be regarded as new and powerful platforms for the fabrication of 2D N-rich carbon-based nanomaterials, which is of great importance for various potential applications.

Crystallization-Induced Morphological Tuning Toward Denim-like Graphene Nanosheets in a KCl-Copolymer Solution.

Although nucleation and crystallization in solution-processed materials synthesis is a natural phenomenon, the morphology design of graphene nanosheets by controlling the dual crystallization has not been established. In this work, we systematically demonstrate how the dual crystallization of ice and potassium chloride induces the morphological variation of the freeze-dried scaffold from fractal structure toward stepped sheet-like structure. A denim-like graphene nanosheet (DGNS) has been fabricated by annealing the F127-coated stepped sheet-like scaffold in nitrogen. DGNS shows parallel and straight stripes with an average stripe spacing of 10 nm. When used as a lithium-ion battery anode, DGNS possesses a superhigh reversible capacity of 1020 mAh g-1 at the current density of 1 A g-1 after 600 cycles. This work reports the control of dual crystallization of ice and salt crystals and provides an efficient way to design the morphology of two-dimensional materials by adjusting the crystallization.