Tailoring Two Polymorphs of LiFePO₄ by Efficient Microwave-Assisted Synthesis: A Combined Experimental and Theoretical Study

LiFePO₄ typically crystallizes in the olivine-type phase (denoted as α-phase hereafter). When high pressure (65 kbar) and elevated temperature (900 °C) are applied, the α-LiFePO₄ transforms into a high-pressure phase (denoted as β-phase hereafter). Here, we report a facile approach to directly tailor the two polymorphs of LiFePO₄ in a controlled way under mild conditions. Employing a microwave-assisted nonaqueous route, highly crystalline LiFePO₄ with either α- or β-phase can be efficiently synthesized within 3 min, by simply tuning the ratio of the solvents, benzyl alcohol, and 2-pyrrolidinone. The resulting β-LiFePO₄ particles exhibit a hierarchical self-assembled bow-tie-like microstructure, whereas the α-phase consists of nanoplates. In addition, the β-phase irreversibly transforms into the α-phase upon heat treatment without alteration of the morphology. After carbon-coating, α-LiFePO₄ and phase-transformed β-LiFePO₄, that is, α-LiFePO₄ with the hierarchical morphology of the β-phase, exhibit excellent electrochemical performance, whereas pristine β-LiFePO₄ displays unfavorable properties. Density functional total energy calculations are performed to get the relative energies and lattice stability of the two phases. A qualitative understanding of the poor electrochemical performance of the β-phase can be deduced from the molecular dynamics of the mobile Li ions in both structures

El Correo gallego : diario político de la mañana: Ano L Número 17779 - 1928 outubro 6

Anatase TiO₂ is among the most studied photocatalytic materials for solar energy conversion and environmental cleanup. However, its poor visible light absorption and high facet-dependent performance limits its utilization. In this study chemical substitution (doping) of TiO₂ nanoparticles with metal ions (Sb, Cr, or Sb/Nb and Cr/Nb) is presented as an alternative strategy to address both issues simultaneously. Highly crystalline doped and codoped TiO₂ nanoparticles were successfully synthesized by a microwave-assisted nonaqueous sol–gel synthesis. The structural and compositional analysis done by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS) showed that depending on the doping applied, variations in particles size and morphology were observed. Doped and codoped samples showed improved absorption in the visible range and in comparison to the undoped TiO₂ displayed improved photocatalytic (PC) activity. The variations of the PC activity, observed among different samples, are attributed to the effect of doping on (i) particles size/morphology, (ii) optical activity, and (iii) on the surface potential differences for the various crystal facets. We found that Sb-doping in TiO₂ diminishes the surface potential difference for {101} reductive and {001} oxidative sites, which makes all crystal surfaces equally attractive to both electrons and holes. Accordingly, in Sb-doped TiO₂ nanoparticles the photocatalytic activity is independent of the exposed crystal facets and thus on the particle morphology. This observation also explains the superior PC performance of this material

A General Method of Fabricating Flexible Spinel-Type Oxide/Reduced Graphene Oxide Nanocomposite Aerogels as Advanced Anodes for Lithium-Ion Batteries

High-capacity anode materials for lithium ion batteries (LIBs), such as spinel-type metal oxides, generally suffer from poor Li⁺ and e^– conductivities. Their drastic crystal structure and volume changes, as a result of the conversion reaction mechanism with Li, severely impede the high-rate and cyclability performance toward their practical application. In this article, we present a general and facile approach to fabricate flexible spinel-type oxide/reduced graphene oxide (rGO) composite aerogels as binder-free anodes where the spinel nanoparticles (NPs) are integrated in an interconnected rGO network. Benefiting from the hierarchical porosity, conductive network and mechanical stability constructed by interpenetrated rGO layers, and from the pillar effect of NPs in between rGO sheets, the hybrid system synergistically enhances the intrinsic properties of each component, yet is robust and flexible. Consequently, the spinel/rGO composite aerogels demonstrate greatly enhanced rate capability and long-term stability without obvious capacity fading for 1000 cycles at high rates of up to 4.5 A g^–1 in the case of CoFe₂O₄. This electrode design can successfully be applied to several other spinel ferrites such as MnFe₂O₄, Fe₃O₄, NiFe₂O₄ or Co₃O₄, all of which lead to excellent electrochemical performances

Aliovalent Ni in MoO₂ Lattice Probing the Structure and Valence of Ni and Its Implication on the Electrochemical Performance

Here, we present a synthesis of MoO₂ nanoparticles doped with 2 at% of Ni in a mixture of acetophenone and benzyl alcohol at 200 °C. Based on <i>in situ</i> X-ray absorption near-edge structure (XANES) and <i>ex situ</i> extended X-ray absorption fine structure (EXAFS) measurements at Ni K-edge and Mo K-edge, we discuss scenarios on how the “doping” reaction, that is, the incorporation of Ni in the MoO₂, proceeds. We can clearly exclude the formation of NiO or Ni nanoparticles. Moreover, within the resolution of our <i>in situ</i> XANES experiments, we observe that the ternary compound Ni:MoO₂ nucleates directly in the final composition. Although the local structure around the Ni ion adopts the MoO₂ crystal structure pointing at the substitution of tetravalent Mo by Ni, we find that Ni remains divalent. This aliovalent substitution results in the relaxation of the local structure, which is additionally reflected in the slight shrinking of the total volume of the unit cell of Ni:MoO₂. Interestingly, such a small amount of divalent Ni has a tremendous effect on the performance of the material as anode in Li-ion batteries. The initial discharge capacity of Ni:MoO₂ based anodes almost doubles from 370 mAh/g for MoO₂ to 754 mAh/g for Ni:MoO₂ at 0.1 C (1 C = 300 mA/g). Additionally, we observed an atypical increase of capacity for both MoO₂ and Ni:MoO₂ anodes upon cycling with increasing cycling rate