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In this work, a new fluorophosphate cathode material Na5V(PO4)2F2 for sodium-ion batteries is synthesized by a facile synthetic strategy for the first time. X-ray diffraction analyses indicate that...

Na2 FePO4 F is a promising cathode material for Na-ion batteries owing to its relatively high discharge voltage and excellent cycling performance. Now, the long- and short-range structural evolution of Na2 FePO4 F during cycling is studied by in situ high-energy X-ray diffraction (XRD), ex situ solid-state nuclear magnetic resonance (NMR), and first-principles DFT calculations. DFT calculations suggest that the intermediate phase, Na1.5 FePO4 F, adopts the space group of P21 /c, which is a subgroup (P21 /b11, No. 14) of Pbcn (No. 60), the space group of the starting phase, Na2 FePO4 F, and this space group provides a good fit to the experimental XRD and NMR results. The two crystallographically unique Na sites in the structure of Na2 FePO4 F behave differently during cycling, where the Na ions on the Na2 site are electrochemically active while those on the Na1 site are inert. This study determines the structural evolution and the electrochemical reaction mechanisms of Na2 FePO4 F in a Na-ion battery.

A new compound Na3V3(PO4)4 is successfully synthesized for sodium ion batteries using a sol–gel method. Composition analysis through ICP-OES confirms the stoichiometry of Na3V3(PO4)4. Structural analysis based on XRD reveals that the new material crystallizes in a monoclinic system with a C2/c space group. The new compound exhibits a layered structure containing 3D Na+ ion channels allowing excellent cycling and rate performance. Even at a high current rate of 3C (1C = 45 mA/g), it still delivers 82% of the theoretical capacity. Meanwhile, 92% of its capacity is retained after 100 electrochemical cycles. The voltage profiles of Na3V3(PO4)4 show that it can reversibly uptake nearly one Na+ ion with a 3.9 V voltage plateau, which is the highest value among Na-containing V-based orthophosphates ever reported.

GeTe is a fundamental material for phase-change memory, one of the most promising next-generation data storage devices. Doping GeTe with W has achieved both high writing (crystallization) speed at elevated temperature and long data retention (amorphous stability) at room temperature, which overcame a key challenge for phase-change memory. Yet the effect of W on amorphous GeTe remains ambiguous at atomic and electronic scales. By means of ab initio calculations and molecular dynamics (AIMD) simulations, we shed light on this issue and reveal that W would agglomerate during the melt-quench process and the chemically bonded W cluster plays a key role in tuning the overall phase-change performances of GeTe. Furthermore, the strong W-Ge and W-Te bonds show vital impact on the local structure and crystallization of amorphous GeTe as well. The present work provides valuable clues for advancing the understanding of transition metals doping effects on amorphous phase-change materials, and hence promotes the development of novel phase-change alloys with improved information storage performance.

Rechargeable sodium-ion batteries (SIBs) are receiving intense interest because the resource abundance of sodium and its lithium-like chemistry make them low cost alternatives to the prevailing lithium-ion batteries in large-scale energy storage devices. Two typical classes of materials including transition metal oxides and polyanion compounds have been under intensive investigation as cathodes for SIBs; however, they are still limited to poor stability or low capacity of the state-of-art. Herein, we report a low cost carbon-coated Na2FeSiO4 with simultaneous high capacity and good stability, owing to the highly pure Na-rich triclinic phase and the carbon-incorporated three-dimensional network morphology. The present carbon-coated Na2FeSiO4 demonstrates the highest reversible capacity of 181.0 mAh g–1 to date with multielectron redox reaction that occurred among various polyanion-based SIBs cathodes, which achieves a close-to-100% initial Coulombic efficiency and a stable cycling with 88% capacity retenti...

Sodium orthosilicates Na2MSiO4 (M denotes transition metals) have attracted much attention due to the possibility of exchanging two electrons per formula unit. In this work, we report a group of sodium iron orthosilicates Na2FeSiO4. Their crystal structures are characterized by a diamond-like Fe-Si network. The Fe-Si network is quite robust against the charge/discharge process, which explains the high structural stability observed in experiment. Furthermore, using the density functional theory within the GGA + U framework and X-ray diffraction studies, the crystal structures and structural stabilities during the sodium extraction/re-insertion process are systematically investigated.

A new cubic polymorph of sodium iron silicate, Na2FeSiO4, is reported for the first time as a cathode material for Na-ion batteries. It adopts an unprecedented cubic rigid tetrahedral open framework structure, i.e., F4̅3m, leading to a polyanion cathode material without apparent cell volume change during the charge/discharge processes. This cathode shows a reversible capacity of 106 mAh g(-1) and a capacity retention of 96% at 5 mA g(-1) after 20 cycles.

A series of lithium borate added electrolyte compounds, xLi3BO3-(1−x)Li3.5Si0.5P0.5O4 (0 ≤ x ≤ 0.2), are synthesized and characterized. This so-called LISICON electrolyte system is analyzed by using X-ray diffraction (XRD), scanning electron microscopy (SEM), solid state nuclear magnetic resonance (ss-NMR), electrochemical impedance spectra (EIS), and direct current (DC) polarization methods. From 11B MAS NMR spectra, it is demonstrated that a small fraction of boron exists in the form of BO4, while its majority settles at grain boundaries in the form of BO3, indicating that lithium borate glasses play a role as sintering assistant. This prominently increases the relative density of samples, and is beneficial to the ionic conductivity. Further results show that the electrical conduction of lithium borate added LISICONs is dominated by Li+ ions, with a transference number of tLi+ ≥ 0.996 and a corresponding ionic conductivity of about 6.5 × 10−6 S cm−1 at room temperature, almost two times that of pristine Li3.5Si0.5P0.5O4.

The cubic garnet-type solid electrolyte Li7La3Zr2O12 with aliovalent doping exhibits a high ionic conductivity, reaching up to ∼10–3 S/cm at room temperature. Fully understanding the Li+ transport mechanism including Li+ mobility at different sites is a key topic in this field, and Li7–2x–3yAlyLa3Zr2–xWxO12 (0 ≤ x ≤ 1) are selected as target electrolytes. X-ray and neutron diffraction as well as ac impedance results show that a low amount of aliovalent substitution of Zr with W does not obviously affect the crystal structure and the activation energy of Li+ ion jumping, but it does noticeably vary the distribution of Li+ ions, electrostatic attraction/repulsion, and crystal defects, which increase the lithium jump rate and the creation energy of mobile Li+ ions. For the first time, high-resolution NMR results show evidence that the 24d, 96h, and 48g sites can be well-resolved. In addition, ionic exchange between the 24d and 96h sites is clearly observed, demonstrating a lithium transport route of 24d–96h–...

Nonspherical hollow nanoparticles (NHNPs) have attracted a great deal of attention in recent years due to their unique properties and many promising applications. However, compared to hollow spheres, the fabrication of NHNPs is generally much more difficult and there are only a few successful examples to date. In this work, X-shaped hollow α-FeOOH penetration twins were first synthesized by a facile hydrothermal reaction. X-shaped α-Fe2O3 hollow nanostructures with high-index { 1 1 2 ¯ 3 } facets exposed were further obtained via the topotactic transformation of α-FeOOH precursor. To the best of our knowledge, this is the first report on the nanostructures with high-index facets as well as a hollow interior. Owing to the special hollow structure and the high-energy surface, the as-obtained α-Fe2O3 nanocrystals show excellent visible-light photocatalytic activity toward the degradation of RhB.

Rechargeable sodium-ion batteries (SIBs) are receiving intense interest because the resource abundance of sodium and its lithium-like chemistry make them low cost alternatives to the prevailing lithium-ion batteries in large-scale energy storage devices. Two typical classes of materials including transition metal oxides and polyanion compounds have been under intensive investigation as cathodes for SIBs; however, they are still limited to poor stability or low capacity of the state-of-art. Herein, we report a low cost carbon-coated Na

The cubic garnet-type solid electrolyte Li7La3Zr2O12 with aliovalent doping exhibits a high ionic conductivity. However, the synergistic effects of aliovalent co-doping on the ionic conductivity of garnet-type electrolytes have rarely been examined. In this work, the synergistic effects of co-dopants Al and Te on the ionic conductivity of garnets were investigated using X-ray diffraction (XRD), 27Al/6Li Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR), Energy Dispersive X-ray Spectroscopy (EDS), Neutron Powder Diffraction (NPD) and Alternating Current (AC) impedance measurements. It was shown that co-dopants Al and Te stabilized the cubic lattice of Li7−2x−3yAlyLa3Zr2−xTexO12 with specific Al/Te ratios, where additional Al had to be included in the structure if the amount of doped Te content x was below 0.5. In the Al and Te co-doped crystal structure, Al was incorporated into the tetrahedral 24d sites of lithium and Te occupied 16a sites of Zr. It was revealed that the occupancy of the latter could suppress the insertion of Al. High-resolution 6Li MAS NMR was able to differentiate the two lithium sites of interest in the garnet structure. Furthermore, it was shown that the mobility of Li ions at 24d sites mainly determined the bulk conductivities of garnet-type electrolytes.

The reactions and structural evolution of FeF3 during cell cycling are investigated in an in situ cell by using Fe K-edge X-ray absorption fine-structure (XAFS) spectroscopy. The results of X-ray absorption near-edge structure spectroscopic analysis demonstrate that there are three stages in the reaction of FeF3 with Li: (1) a two-phase intercalation reaction in the range of x = 0 to 0.46 Li, (2) a single-phase intercalation reaction in the range of x = 0.46 to 0.92 Li, and (3) a conversion reaction in the range of x = 0.92 to 2.78 Li. The coordination numbers (CNs) and bond lengths of the Fe–F bonds or Fe–Fe bonds for the lithiated FeF3 are obtained by performing XAFS fitting. The splitting trends of the Fe–F bond lengths and the Fe–F CNs in the range of x = 0 to 0.92 Li support the proposal that R-3c-structured FeF3 is transformed into R3c-structured Li0.92FeF3 after the intercalation of 0.92 equiv. of Li, and that the intermediate Li0.46FeF3 may be R3-structured. The small Fe–Fe CN of Li2.78FeF3 indica...

The electrochemical mechanism of the cathode material Li2FeSiO4 with reversible extraction/insertion of more than one Li+ from/into the structure has been studied by techniques of in situ synchrotron X-ray absorption near edge structure (XANES) and X-ray diffraction (XRD). These advanced techniques provide effective solutions to address the limitations of characterization by traditional ex situ methods. The study of in situ Fe K-edge XANES indicates that the Fe ion in the Li2FeSiO4 is oxidized continuously to high valence during the charging process from open circuit potential to 4.8 V, which contributes to the high reversible capacities of the materials. In situ XRD and theoretical study from first-principles calculations have been employed to reveal the structural evolution of the Li2FeSiO4 underlying the high capacity during the charge/discharge process. The results of both experimental and theoretical studies are consistent and indicate that Li2FeSiO4 undergoes two two-phase reactions when the electro...

Single-crystal silicon nanowires with the prism structures were synthesized by chemical vapor deposition of SiH 4 gas at 450 °C. Fe particles which were located at the tip of the CNTs were employed as a catalyst for the growth of silicon nanowires (SiNWs). Transmission electron microscopy studies of the materials showed that the nanowires have a diameter of 50–70 nm and a length of several micrometers. High-resolution transmission electron microscopy demonstrated that the nanowires have excellent single-crystal characteristics. Both the CNTs and Fe play a key role in the growth process of the SiNWs. A growth mechanism was proposed for the growth of silicon nanowires under our experimental conditions.

Hematite (α-Fe2O3) concave nanocubes bound by high-index {1344̄} and {123̄8} facets were synthesized and their catalytic activity for CO oxidation were also investigated.

A Li2FeSiO4/C composite material has been prepared via a solution-polymerization approach. The composite is characterized by X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), scanning electron microscope (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and superconducting quantum interference device (SQUID). The electrochemical performance of the Li2FeSiO4 is greatly enhanced and the initial discharge capacity is ∼220 mA h g−1, when it is cycled between 1.5–4.8 V. This indicates that more than one lithium ion can be extracted out of the Li2FeSiO4 lattice. At high current densities, the Li2FeSiO4/C also exhibits excellent rate capability and cycling stability. This indicates that it is a very promising cathode material for next generation lithium-ion batteries.