Microwave-Assisted Solvothermal Synthesis and Characterization of Metastable LiFe1−x(VO)xPO4 Cathodes

New Cathode Materials in the Fe-PO4 -F Chemical Space for High-Performance Sodium-Ion Storage

Sodium and iron make up the perfect combination for the growing demand for sustainable energy storage systems, given the natural abundance and sustainability of the two building block elements. However, most sodium-iron electrode chemistries are plagued by intrinsic low energy densities with continuous ongoing efforts to solve this. Herein, the chemical space of a series of (meta)stable, off-stoichiometric Fe-PO4 -F phases is analyzed. Some are found to display markedly improved electrochemical activity for sodium storage, as compared to the amorphous or thermodynamically stable phases of equivalent composition. The metastable crystalline Na1.2 Fe1.2 PO4 F0.6 delivers a reversible capacity of more than 140 mAh g-1 with an average discharge potential of 2.9 V (vs Na+ /Na0 ) resulting in a practical specific energy density of 400 Wh kg-1 (estimated at the material level), outperforming many developed Fe-PO4 analogs thus far, with further multiple possibilities to be explored toward improved energy storage metrics. Overall, this study unlocks the possibilities of off-stoichiometric Fe-PO4 -F cathode materials and reveals the importance to explore the oft-overlooked metastable or transient state materials for energy storage.

Improved rate capability and cycling stability of bicontinuous hierarchical mesoporous LiFePO4/C microbelts for lithium-ion batteries

Bicontinuous hierarchical mesoporous LiFePO₄/C microbelts have been synthesized using a simple dual-solvent electrospinning method for the first time. The sample exhibits a high reversible capacity (153 mA h g⁻¹ at 0.5C), and an excellent high rate cycling performance.

Morphology and surface properties of LiVOPO4: a first principles study

First principles calculations were used to investigate the surface energies, equilibrium morphology, surface redox potentials, and surface electrical conductivity of LiVOPO4. Relatively low-energy surfaces are found in the (100), (010), (001), (011), (111), and (201) orientations of the orthorhombic structure. Thermodynamic equilibrium shape of the LiVOPO4 crystal is built with the calculated surface energies through a Wulff construction. The (001) and (111) orientations are the dominating surfaces in the Wulff shape. Similar calculations for VOPO4 display a larger decrease in surface energies for the (100) surface rather than those in the other surfaces. It suggests that the Wulff shape of LiVOPO4 is closely related to the chemical environment around. Surfaces (100), (010) and (201) present lower Li surface redox potentials in comparison with the bulk material. Therefore, the Li migration rate on surfaces could be effectively increased by maximizing the exposure of these low redox potential surfaces. In addition, lower surface band gaps are found in all orientations compared to the bulk one, which indicates that electrical conductivity can be improved significantly by enlarging surfaces with relatively low band gaps in the particle. Therefore, synthesizing (201) and (100) nanosheets will greatly improve the electrochemical properties of the material.

Microwave-hydrothermal synthesis and characterization of nanostructured copper substituted ZnM2O4 (M = Al, Ga) spinels as precursors for thermally stable Cu catalysts

Nanostructured Cu(x)Zn(1-x)Al(2)O(4) with a Cu:Zn ratio of ¼:¾ has been prepared by a microwave-assisted hydrothermal synthesis at 150 °C and used as a precursor for Cu/ZnO/Al(2)O(3)-based catalysts. The spinel nanoparticles exhibit an average size of approximately 5 nm and a high specific surface area (above 250 m(2) g(-1)). Cu nanoparticles of an average size of 3.3 nm can be formed by reduction of the spinel precursor in hydrogen and the accessible metallic Cu(0) surface area of the reduced catalyst was 8 m(2) g(-1). The catalytic performance of the material in CO(2) hydrogenation and methanol steam reforming was compared with conventionally prepared Cu/ZnO/Al(2)O(3) reference catalysts. The observed lower performance of the spinel-based samples is attributed to a lack of synergetic interaction of the Cu nanoparticles with ZnO due to the incorporation of Zn(2+) in the stable spinel lattice. Despite its lower performance, however, the nanostructured nature of the spinel catalyst was stable after thermal treatment up to 500 °C in contrast to other Cu-based catalysts. Furthermore, a large fraction of the re-oxidized copper migrates back into the spinel upon calcination of the reduced catalyst, thereby enabling a regeneration of sintered catalysts after prolonged usage at high temperatures. Similarly prepared samples with Ga instead of Al exhibit a more crystalline catalyst with a spinel particle size around 20 nm. The slightly decreased Cu(0) surface area of 3.2 m(2) g(-1) due to less copper incorporation is not a significant drawback for the methanol steam reforming.