Maciej Nowagiel

Several bismuth sesquioxide (Bi $$_{2}$$2 O $$_{3}$$3 ) phases (namely $$\upbeta$$β , $$\upgamma$$γ and $$\updelta$$δ -like) were previously stabilized at room temperature by nanocrystallization of bismuthate glasses. Normally, a monoclinic $$\upalpha$$α phase is the stable one at ambient conditions in polycrystalline materials. In this work, we wanted to observe if any phase transitions would occur below the room temperature for $$\upbeta$$β , $$\upgamma$$γ and $$\updelta$$δ -like phases confined in a residual glassy matrix. Observations were made down to 100 K using X-ray diffractometry (XRD) and differential scanning calorimetry (DSC). Both experimental techniques showed no traces of phase transition upon cooling to the temperature limit of our equipment. From XRD studies, the lattice parameter of the δ-like phase was determined. The values were compared to the values calculated by molecular dynamics studies of overcooled $$\updelta$$δ -Bi $$_{2}$$2 O $$_{3}$$3 phase.

Recently, it was shown that the nanocrystallization of $\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$ glasses with the addition of $\mathrm{S}\mathrm{i}{\mathrm{O}}_2$ and $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ leads to the stabilization of the $\delta$-like $\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$ phase at least down to room temperature, which is significantly below its stability range in bulk form. In this research, we investigated the properties of bismuthate glasses synthesized with various glass-forming agents such as $\mathrm{S}\mathrm{i}{\mathrm{O}}_2$, $\mathrm{G}\mathrm{e}{\mathrm{O}}_2$, ${\mathrm{B}}_2{\mathrm{O}}_3$, and $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$. It was demonstrated that vitrification of all these systems is possible using a standard melt quenching route. Furthermore, we investigated the crystallization processes in pristine glasses upon increasing the temperature and the thermal stability of arising phases using thermal analysis and high-temperature XRD in situ experiments. It was shown that it is possible to stabilize crystallites’ isostructures with $\delta$-$\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$ embedded in a residual glassy matrix down to room temperature. The temperature range of the appearance of the $\delta$-like phase strongly depended on the nominal composition of the glasses. We postulate that the confinement effect depends on the local properties of the residual glassy matrix and its ability to introduce sufficient force to stretch the structure of the $\delta$-like $\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$ phase in the nanocrystallites.

Recently, an interest in NASICON-type materials revived, as they are considered potential cathode materials in sodium–ion batteries used in large-scale energy storage. We applied a facile technique of thermal nanocrystallization of glassy analogs of these compounds to enhance their electrical parameters. Six nanomaterials of the Na₃M₂(PO₄)₂F₃ (M = V, Ti, Fe) system were studied. Samples with nominal compositions of Na₃V₂(PO₄)₂F₃, Na₃Ti₂(PO₄)₂F₃, Na₃Fe₂(PO₄)₂F₃, Na₃TiV(PO₄)₂F₃, Na₃FeV(PO₄)₂F₃ and Na₃FeTi(PO₄)₂F₃ have been synthesized as glasses using the melt-quenching method. X-ray diffraction measurements were conducted for as-synthesized samples and after heating at elevated temperatures to investigate the structure. Extensive impedance measurements allowed us to optimize the nanocrystallization process to enhance the electrical conductivity of cathode nanomaterials. Such a procedure resulted in samples with the conductivity at room temperature ranging from $1\times {10}^{-9}$ up to $8\times {10}^{-5}$ S/cm. We carried out in situ impedance spectroscopy measurements (in an ultra-high-frequency range up to 10 GHz) and compared them with thermal events observed in differential thermal analysis studies.

Alluaudite-type materials are systematically attracting more attention as prospective cathode materials for sodium-ion batteries. It has been demonstrated that optimized thermal nanocrystallization of glassy analogs of various cathode materials may lead to a significant increase in their electrical conductivity. In this paper, three alluaudite-like glasses (Na${}_2$Fe${}_3$(PO${}_4$)${}_3$—FFF, Na${}_2$VFe${}_2$(PO${}_4$)${}_3$—VFF, and Na${}_2$VFeMn(PO${}_4$)${}_3$—VFM) were synthesized and subjected to an optimized thermal nanocrystallization. This procedure resulted in nanostructured samples with increased electrical conductivity at room temperature: $5\times {10}^{-7}$ S/cm (FFF), $7\times {10}^{-5}$ S/cm (VFM), and $6\times {10}^{-4}$ S/cm (VFF). The nanocrystalline microstructure was also evidenced by ultra-high-frequency impedance spectroscopy (up to 10 GHz) and proposed electrical equivalent circuits. Prototype electrochemical cells were assembled and characterized with voltage cutoffs of 1.5 and 4.5 V. The electrochemical performance was, however, modest. The gravimetric capacity varied between the studied materials, but did not exceed 35 mAh/g. Capacity retention after ca. 100 cycles was satisfactory. Further optimization of the residual-glass-to-nanocrystallite volume ratio would be desirable.

Alluaudite-type materials are systematically attracting more attention as prospective cathode materials for sodium ion batteries. In this paper, we strove to optimize various synthesis parameters of three alluaudite compositions (Na${}_2$Fe${}_3$(PO${}_4$)${}_3$—FFF, Na${}_2$VFe${}_2$(PO${}_4$)${}_3$—VFF, and Na${}_2$VFeMn(PO${}_4$)${}_3$—VFM) to obtain nanostructured alluaudite-type glass-ceramics with high phase purity. We systematically investigated the role of the chemical reactions, temperature and time of melting, cooling rate, and reducing factors on the quality of the final products. A detailed synthesis protocol along with X-ray diffractometry, thermal analysis, scanning electron microscopy imaging, and electrical conductivity measurements (with impedance spectroscopy) are reported. As a result, a significant increase of the conductivity was observed in the nanomaterials. The highest value was reached for the VFF composition and was equal to $6\times {10}^{-4}$ S/cm at room temperature.