Manipulating amorphous and crystalline hybridization of Na3V2(PO4)3/C for enhancing sodium-ion diffusion kinetics.

Na3V2(PO4)3 (NVP) has attracted considerable attention as a promising cathode material for sodium-ion batteries (SIBs). But its insufficient electronic conductivity, limited capacities, and fragile structure hinder its extended application, particularly in scenarios involving rapid charging and prolonged cycling. A hybrid cathode material has been developed to integrate both amorphous and crystalline phases, with the objective of improving the rate performance and Na storage capacity by leveraging bi-phase coordination. Consequently, the combination of amorphous and crystalline phases enhanced the kinetics of Na-ion diffusion, resulting in a 1-2 orders of magnitude enhancement in diffusion dynamics. Furthermore, the existence of amorphous states has been demonstrated to elevate the active Na2 site content, resulting in an increased reversible capacity. This assertion is substantiated by evidence derived from solid-state nuclear magnetic resonance (ss-NMR) and electrochemical characteristics. The innovative bi-phase collaborative material provides a specific capacity of 114 mAh/g at 0.2 C, exceptional rate performance of 82 mAh/g at 10 C, and remarkable long-term cycle stability, retaining 95 mAh/g at 5 C even after 300 cycles. In conclusion, the homogeneous hybridization of amorphous and crystalline phases presents itself as a promising and effective strategy for improving Na-ion storage capacity of cathodes in SIBs.

Hot-Pressing Enhances Mechanical Strength of PEO Solid Polymer Electrolyte for All-Solid-State Sodium Metal Batteries.

Poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) are widely utilized in all-solid-state sodium metal batteries (ASSSMBs) due to their excellent flexibility and safety. However, poor ionic conductivity and mechanical strength limit its development. In this work, an emerging solvent-free hot-pressing method is used to prepare mechanically robust PEO-based SPE, while sodium superionic conductors Na3 Zr2 Si2 PO12 (NZSP) and NaClO4 are introduced to improve ionic conductivity. The as-prepared electrolyte exhibits a high ionic conductivity of 4.42 × 10-4 S cm-1 and a suitable electrochemical stability window (4.5 V vs Na/Na+ ). Furthermore, the SPE enables intimate contact with the electrode. The Na||Na3 V2 (PO4 )3 @C ASSSMB delivers a high-capacity retention of 97.1% after 100 cycles at 0.5 C and 60 °C, and exhibits excellent Coulombic efficiency (CE) (close to 100%). The ASSSMB with the 20 µm thick electrolyte also demonstrates excellent cyclic stability. This study provides a promising strategy for designing stable polymer-ceramic composite electrolyte membranes through hot-pressing to realize high-energy-density sodium metal batteries.

Multiscale Investigation into Chemically Stable NASICON Solid Electrolyte in Acidic Solutions.

Natrium superionic conductor (NASICON) solid electrolyte has been attracting wide attention due to its high ionic conductivity, low cost, and environmental friendliness. In this work, the chemical stability of the NASICON solid electrolyte with the composition of Na3Zr2Si2PO12 was evaluated in acidic solutions with different pH values, and the corrosion mechanism of the NASICON solid electrolyte was revealed at the multiscale level. Variations in bulk impedance, grain boundary impedance, and surface crack impedance with immersion time were determined by an AC impedance method. Comprehensive studies upon scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) etching, X-ray diffraction (XRD), and Raman spectroscopy, the morphological transformation, degradation limit depth, Cl- penetration effect, and proton exchange between H3O+ and Na+ were examined ranging from macro- and meso- to microscales, respectively. With the decrease of the pH of the solution, the exchange rate between H3O+ and Na+ protons increases. The lack of Na+ within the crystallographic lattice leads to the shrinkage of phosphorus-oxygen tetrahedra, which is the main reason for the decrease of unit cell volume, grain refinement, and surface cracks gradually. This work features multiscale characterizations of crystal structure, grain boundaries, surface morphology changes, and Na+ transport, which deepens our physicochemical understanding of solid electrolytes with high chemical stability.

Plasma tailored reactive nitrogen species in MOF derived carbon materials for hybrid sodium-air batteries.

The rational design of efficient and durable electrocatalysts to accelerate sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics is highly desirable for enhancing the efficiency of fuel cells and metal-air batteries. Here, we demonstrated a low-temperature plasma strategy at atmospheric pressure for enhancing the catalytic activity of metal-organic framework derived N-doped carbon nanotubes (MOF-NCNTs) by changing the relative contents of Co-Nx sites, Co-Co bonds and pyridinic-N. The increase of pyridinic-N/pyrrolic-N ratio improves the ORR performance, while unsaturated Co-Nx sites and strong Co-Co bonds promote the OER performance. The relative contents of pyridinic-N, Co-Nx sites, and Co-Co bonds in MOF-NCNTs can be readily tailored by varying the plasma treatment time. The MOF-NCNTs treated with N2 plasma for 4 min (MOF-NCNTs-N2-4) exhibited improved ORR (ηonset: 0.91 V) and OER (η10: 0.44 V) activities compared to MOF-NCNTs because of the higher ratio of pyridinic-N to pyrrolic-N and higher relative contents of Co-Nx sites and Co-Co bonds. The hybrid sodium-air batteries (HSABs) assembled with MOF-NCNTs-N2-4 catalyst display a low overpotential of 0.35 V and excellent round trip efficiency of 88.9% at 0.1 mA cm-2. Besides, they also exhibited great cycling stability with an average discharge voltage of 2.75 V and an outstanding round trip efficiency of 84% after 150 cycles.