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.

A hybrid solid electrolyte for solid-state sodium ion batteries with good cycle performance.

A ceramic in polymer hybrid solid electrolyte (HSE) based on a poly vinylidene fluoride-hexafluoropropylene (PVDF-HFP) polymer comprising Na3Zr2Si2PO12 (NASICON) ceramic particles was prepared by a simple solution casting method followed by activation in a liquid electrolyte. The prepared HSE exhibits good flexibility, high ionic conductivity of 2.25 × 10-3 S cm-1 at room temperature (RT), and good interface stability. The carbon coated sodium vanadium phosphate (Na3V2(PO4)3/C) cathode was synthesized by the sol-gel method and assembled into batteries with different electrolytes. The batteries based on HSE exhibit better electrochemical performance than that of NASICON ceramic solid electrolytes, which delivers a reversible capacity of 98 mAh · g-1 at 0.2 C and exhibits good capacity retention of 85% after 175 cycles at 0.5 C. Not only does the HSE inherit great flexibility, but also exhibits good interfacial contact with electrodes. The schematic diagram of Na-ion conductivity in ceramic, polymer and HSE was illustrated to demonstrate the sodium ion transport mechanism. The HSE with high ionic conductivity and good flexibility for interfacial contact with electrodes shall provide a designing strategy for different solid-state batteries.

Highly dispersed Co nanoparticles decorated on a N-doped defective carbon nano-framework for a hybrid Na-air battery.

Efficient and low-cost bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are of vital importance in energy conversion. Herein, an excellent highly dispersed Co nanoparticle decorated N-doped defective carbon nano-framework (Co-N-C) derived from a ZnCo bimetal organic framework (bi-MOF) is reported. A high specific surface area originating from zinc evaporation facilitates the adsorption and desorption of oxygen, which promotes the accessibility of catalytic sites. The abundant Co-N-C species act as strong bridging bonds between Co nanoparticles and carbon materials which facilitate interfacial electron transfer. Co-N-C-0.5 (0.5 represents the molar ratio of Zn in the initial ZIF-67) exhibits a low overpotential gap of 0.94 V due to the number of active sites (e.g. N-doped defective carbon and the CoNx/Co composite) and fast interfacial electron transfer. In addition, a hybrid Na-air battery with the Co-N-C-0.5 material displays a low voltage gap of 0.31 V and a high round-trip efficiency of 90.0% at a current density of 0.1 mA cm-2. More importantly, the hybrid Na-air battery shows fantastic cyclability for charging and discharging due to its stable structure. Our results confirm Co-N-C materials derived from a bi-MOF as alternatives to high-cost Pt/C catalysts for ORR and OER activities in metal-air batteries.

Challenges and perspectives of NASICON-type solid electrolytes for all-solid-state lithium batteries.

NASICON-type (lithium super ionic conductor) solid electrolyte is of great interest because of its high ionic conductivity, wide potential window, and good chemical stability. In this paper, the key problems and challenges of NASICON-type solid electrolyte are described from the aspects of ionic conductivity, electrode interface, and electrochemical stability. Firstly, the migration mechanism of lithium ion is analyzed from the three-dimensional structure of NASICON-type solid electrolyte, and progress in the research of conductivity and stability is summarized. Then, the effective methods to reduce interface impedance and improve the cycle stability of all-solid-state lithium batteries (ASSLBs) with NASICON-type solid electrolyte are introduced. Finally, solutions to improve the conductivity of electrolytes and deal with electrode/electrolyte interface problems are summarized, and the development prospects of ASSLBs are discussed.

Preparation of Nickel Nanoparticles by Direct Current Arc Discharge Method and Their Catalytic Application in Hybrid Na-Air Battery.

Nickel nanoparticles were prepared by the arc discharge method. Argon and argon/hydrogen mixtures were used as plasma gas; the evaporation of anode material chiefly resulted in the formation of different arc-anode attachments at different hydrogen concentrations. The concentration of hydrogen was fixed at 0, 30, and 50 vol% in argon arc, corresponding to diffuse, multiple, and constricted arc-anode attachments, respectively, which were observed by using a high-speed camera. The images of the cathode and anode jets were observed with a suitable band-pass filter. The relationship between the area change of the cathode/anode jet and the synchronous voltage/current waveform was studied. By investigating diverse arc-anode attachments, the effect of hydrogen concentration on the features of nickel nanoparticles were investigated, finding that 50 vol% H2 concentration has high productivity, fine crystallinity, and appropriate size distribution. The synthesized nickel nanoparticles were then used as catalysts in a hybrid sodium⁻air battery. Compared with commercial a silver nanoparticle catalyst and carbon black, nickel nanoparticles have better electrocatalytic performance. The promising electrocatalytic activity of nickel nanoparticles can be ascribed to their good crystallinity, effective activation sites, and Ni/NiO composite structures. Nickel nanoparticles prepared by the direct current (DC) arc discharge method have the potential to be applied as catalysts on a large scale.

Removal of B from Si by Hf addition during Al-Si solvent refining process.

A small amount of Hf was employed as a new additive to improve B removal in the electromagnetic solidification refinement of Si with an Al-Si melt, because Hf has a very strong affinity for B. The segregation ratio of Hf between the solid Si and Al-Si melt was estimated to range from 4.9 × 10-6 to 8.8 × 10-7 for Al concentrations of 0 to 64 at.%, respectively. The activity coefficient of Hf in solid Si at its infinite dilution was also estimated. A small addition of Hf (<1025 parts per million atoms, ppma) significantly improved the B removal. It was confirmed that the use of an increased Hf addition, slower cooling rate, and Al-rich Al-Si melt as the refining solvent removed B more efficiently. B in Si could be removed as much as 98.2% with 410 ppma Hf addition when the liquidus temperature of the Al-Si melt was 1173 K and the cooling rate was 4.5-7.6 K min-1. The B content in Si could be controlled from 153 ppma to 2.7 ppma, which meets the acceptable level for solar-grade Si.

Influence of vacuum upon preparation and luminescence of Si4+ and Ti4+ codoped Gd2O2S:Eu phosphor.

As a novel red long afterglow phosphor, Si(4+) and Ti(4+) ion codoped Gd2O2S:Eu phosphor with spherical morphology, sub-micrometer size and narrow particle size distribution was synthesized by solid-state reaction in vacuum. The vacuum synthesis mechanism was determined by thermal analysis. The crystal structure, luminescence properties and mechanisms were investigated respectively by XRD, SEM and fluorescence spectrophotometer. The results show that well-crystallized Gd2O2S:Eu,Si,Ti phosphors are of hexagonal structure which is in agreement with the standard powder peak positions of Gd2O2S hexagonal phase. It displays pure red emission because of the strongest peaks at 627nm and 617nm which are attributed to energy transfer ((5)D0-(7)F2). There is a little blue shift of charge transfer excitation band in the excitation spectra between the bulk and sub-micrometer-sized samples, which may stem from size dependent shift and different lattice distortion in the position of the Eu(3+)-ligand electron transfer absorption/excitation band. To further study the influence of the impurities in Gd2O2S:Eu crystals on crystal growth, the simulated crystal face and its XRD patterns were illustrated. The preferred orientation of crystal growth changed from crystal face (101) to (100) thus to result in different luminescence mechanisms.

Silica nanoparticles functionalized via click chemistry and ATRP for enrichment of Pb(II) ion.

Silica nanoparticles have been functionalized by click chemistry and atom transfer radical polymerization (ATRP) simultaneously. First, the silanized silica nanoparticles were modified with bromine end group, and then the azide group was grafted onto the surface via covalent coupling. 3-Bromopropyl propiolate was synthesized, and then the synthesized materials were used to react with azide-modified silica nanoparticles via copper-mediated click chemistry and bromine surface-initiated ATRP. Transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis were performed to characterize the functionalized silica nanoparticles. We investigated the enrichment efficiency of bare silica and poly(ethylene glycol) methacrylate (PEGMA)-functionalized silica nanoparticles in Pb(II) aqueous solution. The results demonstrated that PEGMA-functionalized silica nanoparticles can enrich Pb(II) more quickly than pristine silica nanoparticles within 1 h.

Synthesis and Photoluminescence Property of Silicon Carbide Nanowires Via Carbothermic Reduction of Silica.

Silicon carbide nanowires have been synthesized at 1400 degrees C by carbothermic reduction of silica with bamboo carbon under normal atmosphere pressure without metallic catalyst. X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, transmission electron microscopy and Fourier transformed infrared spectroscopy were used to characterize the silicon carbide nanowires. The results show that the silicon carbide nanowires have a core-shell structure and grow along <111> direction. The diameter of silicon carbide nanowires is about 50-200 nm and the length from tens to hundreds of micrometers. The vapor-solid mechanism is proposed to elucidate the growth process. The photoluminescence of the synthesized silicon carbide nanowires shows significant blueshifts, which is resulted from the existence of oxygen defects in amorphous layer and the special rough core-shell interface.