Synthesis and luminescent properties of novel Ba2−xEuxZr2−yHfySi3O12 phosphor

A series of Eu²⁺ activated luminescent materials according to the composition of Ba_2−xEu_xZr_2−yHf_ySi₃O₁₂ were synthesized using a high temperature solid-state reaction method starting from metal oxides and carbonates. Single phase powders were obtained using two annealing steps and boric acid as a flux. Firstly, starting materials were sintered at 1450 °C for 5 h under CO atmosphere and subsequently annealed at 1200 °C for 5 h under N₂/H₂ (95%/5%) gas flow. All samples were characterized by powder X-ray diffraction (XRD) analysis, thermal quenching (TQ), fluorescence lifetime measurements and photoluminescence (PL) techniques. Moreover, emission colour points, luminous efficacies and quantum efficiencies (QE) were calculated and discussed as a function of Eu²⁺ concentration and Zr/Hf ratio of the host lattice.     

Impact of synthesis conditions on the structure and performance of Li2FeSiO4

Three different synthesis techniques (hydrothermal synthesis, modified Pechini synthesis and Pechini synthesis) were successfully used for preparation of Li₂FeSiO₄ samples. The obtained samples possess some differences in the morphology and in the particle size, as well as in the presence of in situ formed carbon. The best electrochemical performance has been obtained with the smallest particles embedded into carbon matrix. Such a Li₂FeSiO₄/C composite contains the highest amounts of impurities (Fe₂O₃, SiO₂ and Li₂SiO₃) and only 68.8 at.% of iron is in the form of Fe^II as detected by Mössbauer spectroscopy, respectively. Despite the highest amount of impurities, the sample shows the highest reversible capacity (approximately 100 mAh g⁻¹ based on whole silicate-derived material). With the proper structuring of Li₂FeSiO₄/C composites, utilisation of large part of capacity is also possible at current densities corresponding to C/5 and C/2 cycling rate. A lower amount of impurities was found in the samples that do not contain any in situ carbon after synthesis. Among them, the highest purity is possessed by the sample prepared at 900 °C, as determined using Mössbauer spectroscopy. The results obtained by Mössbauer spectroscopy and XRD analysis indicate on the differences in the crystal structure between the thermally treated samples and the sample prepared by hydrothermal synthesis.     

Li2MSiO4 (M = Fe and/or Mn) cathode materials

Two iso-structural end members of the family of orthosilicates, i.e. Li₂MSiO₄ (M = Mn and Fe) and their solid solutions, were prepared and electrochemically characterized for potential use in Li-ion batteries. Due to the low specific conductivity (∼5 × 10⁻¹⁶ S cm⁻¹ for Li₂MnSiO₄ and ∼6 × 10⁻¹⁴ S cm⁻¹ for Li₂FeSiO₄ at room temperature), small particles in an intimate contact with a conducting phase (i.e. carbon) are needed. Li₂MSiO₄/C composites (M = Mn and/or Fe) prepared by the Pechini synthesis generally leads to 30–50 nm large particles embedded in a carbon matrix. The amount of carbon in the composite is close to 10 wt.% for the Li₂FeSiO₄/C composite and slightly more than 5 wt.% for the Li₂MnSiO₄/C composite. In situ XRD experiment confirms a structural collapse of Li₂MnSiO₄ and the observed structural stability is completely different for Li₂FeSiO₄, which undergoes a fully reversible two-phase transition. Solid solutions between Li₂MnSiO₄ and Li₂FeSiO₄ in principle lead to higher capacities (>1e⁻ per transition metal is feasible). For a long-term operation the cut-off voltage should be properly chosen. Electrochemical characterisation and in situ XRD experiments suggest the use of cut-off voltage close to 4.2 V.