Porous N-Doped Carbon Decorated with Atomically Dispersed Independent Dual Metal Sites from Energetic Zeolite Imidazolate Frameworks as Bidirectional Catalysts for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have ultrahigh theoretical specific capacity and energy density, which are considered to be very promising energy storage devices. However, the slow redox kinetics of polysulfides are the main reason for the rapid capacity decay of Li-S batteries. A reasonable electrocatalyst for the Li-S battery should reduce the reaction barrier and accelerate the reaction kinetics of the bidirectional catalytic conversion of lithium polysulfides (LiPSs), thereby reducing the cumulative concentration of LiPSs in the electrolyte. In this report, porous N-doped carbon nanofibers decorated with independent dual metal sites as catalysts for Li-S batteries were fabricated in one step using a fusion-foaming method. Experimental and theoretical analyses demonstrate that the synergistic effect of independent dual metal sites provides strong LiPS affinity, improved electronic conductivity, and enhanced redox kinetics of polysulfides. Therefore, the assembled Li-S battery exhibits high rate performance (discharge specific capacity of 771 mA h g-1 at 2C) and excellent cycle stability (capacity decay rate of 0.51% after 1000 cycles at 1C).

MnFe2O4@C Nanofibers as High-Performance Anode for Sodium-Ion Batteries

MnFe2O4 nanodots (∼3.3 nm) homogeneously dispersed in porous nitrogen-doped carbon nanofibers (denoted as MFO@C) were prepared by a feasible electrospinning technique. Meanwhile, MFO@C with the character of flexible free-standing membrane was directly used as binder- and current collector-free anode for sodium-ion batteries, exhibiting high electrochemical performance with high-rate capability (305 mA h g(-1) at 10000 mA g(-1) in comparison of 504 mA h g(-1) at 100 mA g(-1)) and ultralong cycling life (ca. 90% capacity retention after 4200 cycles). The Na-storage mechanism was systematically studied, revealing that MnFe2O4 is converted into metallic Mn and Fe after the first discharge (MnFe2O4 + 8Na(+) + 8e(-) → Mn + 2Fe + 4Na2O) and then to MnO and Fe2O3 during the following charge (Mn + 2Fe + 4Na2O → MnO + Fe2O3 + 8Na(+) + 8e(-)). The subsequent cycles occur through reversible redox reactions of MnO + Fe2O3 + 8Na(+) + 8e(-) ↔ Mn + 2Fe + 4Na2O, of which the reduction/oxidation of MnO/Mn takes place at a lower potential than that of Fe2O3/Fe. Furthermore, a soft package sodium-ion full battery with MFO@C anode and Na3V2(PO4)2F3/C cathode was assembled, delivering a stable capacity of ∼400 mA h g(-1) for MFO@C (with 100 cycles at 500 mA g(-1)) and a promising energy density of 77.8 Wh kg(-1) for the whole battery. This is owing to the distinctive structure of very-fine MnFe2O4 nanodots embedded in porous N-doped carbon nanofibers, which effectively improves the utilization rate of active materials, facilitates the transportation of electrons and Na(+) ions, and prevents the particle pulverization/agglomeration upon prolonged cycling.

Tin Nanodots Encapsulated in Porous Nitrogen-Doped Carbon Nanofibers as a Free-Standing Anode for Advanced Sodium-Ion Batteries

Ultrasmall Sn nanodots (1-2 nm) are homogeneously encapsulated in porous N-doped carbon nanofibers using a simple and scalable electrospinning method. The composite nanofibers weave into flexible free-standing membrane and can be directly used as binder- and current collector-free anode for Na-ion batteries, exhibiting excellent electrochemical performance with high reversible capacity, exceptional rate capability, and ultralong cycle life.