Jerzy Edward Garbarczyk

The report presents the electrical, structural, and microstructural properties of high-pressure–high-temperature-treated (HPHT) composites composed of δ-like Bi₂O₃ nanograins embedded in an aluminosilicate glassy matrix. Nanocomposites were obtained by heat treatment of the Bi₂O₃-Al₂O₃-SiO₂ ternary glass system, followed by high-pressure molding (above 750 MPa). The total oxygen conductivity σ_t of the studied nanocomposites was high and approached a value of 4.5 × 10⁻⁴ S/cm at 600 °C. Due to HPHT treatment, we could also determine the intragrain conductivity of δ-Bi₂O₃ nanocrystallites. In this case, the value of σ_δ was even higher and was equal to 1.3 × 10⁻³ S/cm at 600 °C. It was also possible to study the temperature dependence of intragrain conductivity, showing two activation energies, which probably reflect the order–disorder transition within the sublattice of mobile O²⁻ ions. The obtained nanocomposites exhibited promising properties for applications in electrochemical devices operating in the intermediate temperature range from 300 to 600 °C.

Olivine-like NaFePO4 glasses and nanocomposites are promising materials for cathodes in sodium batteries. Our previous study concerned the influence of high pressure on DC electric conductivity of NaFePO4 glasses. This work presents the results of AC measurements of glasses and nanocomposites from pressure treatment. Depending on the temperature range, the electronic conduction mechanism changed from variable range hopping (between Fe2+/Fe3+centers) to phonon-assisted hopping. For DC electric conductivity the portrayal via the classic Mott’s Variable Range Hopping is limited to low temperatures. Spectra of AC conductivity for glasses and pressure-formed nanocomposites fulfill Jonscher’s scaling law with power exponents ranging from 0.3 to 0.9 depending on temperature and the electron hopping mechanism. Due to the decrease in distance between hopping centers, an increase of AC conductivity around 2-3 orders of magnitude was observed after high pressure-high temperature treatment. Up to 200 K temperature dependencies of AC conductivity follow the pattern suggested by Mott's theory. Above that temperature corresponds to phonon-assisted hopping with activation energies EAC≈0.40 eV (for glass: amorphous solid) and EAC≈0.18 eV (for glass + nanocrystallites nanocomposite). Notable that they are considerably lower than values of EDC for DC conductivity. Results obtained show that high-temperature compressing, just below the glass temperature, significantly leads to a stable decrease in distance between hopping centers related to nanocrystallites in a glassy matrix. Consequently, it leads to an essential rise in electric conductivity and volumetric capacity. AC spectra of electronic conductivity are disturbed by the conduction of Na+ mobile ions, which impacts polaron conductivity and can be a limiting factor for conductivity.

New nanocomposite material was prepared by high pressure processing of starting glass of nominal composition NaFePO$_{4}$. Thermal, structural, electrical and dielectric properties of the prepared samples were studied by differential thermal analysis (DTA), X-ray diffraction (XRD) and broadband dielectric spectroscopy (BDS). It was demonstrated that high pressure – high temperature treatment (HPHT) led to an increase of electrical conductivity of initial glasses by 2 orders of magnitude. It was also shown that the observed effect was stronger than for lithium analogue of this material studied by us earlier. The observed enhancement of conductivity was explained by Mott’s theory of electron hopping, which is more frequent in sample after pressure treatment. The final composite consisted of nanocrystalline NASICON and alluaudite phases, which are electrochemically active in potential cathode materials for Na batteries. Average dimensions of crystallites estimated from XRD studies were between 40 and 90 nm, depending on the phase. Some new aspects of local dielectric relaxations in studied materials were also discussed. It was shown that a combination of high pressures and BDS method is a powerful method to study relaxation processes and molecular movements in solids. It was also pointed out that high-pressure cathode materials may exhibit higher volumetric capacities compared with commercially used cathodes with carbon additions.

This study demonstrated for the first time that it is possible to prepare nanocrystalline δ-Bi₂O₃ that is stable at room temperature by twin-rollers and free cooling methods, using a ceramic crucible. The phase composition of prepared samples and upper limit of the thermal stability of nanograins confined in an amorphous matrix were determined by the X-ray diffraction (XRD) method. The average size of crystallites and the microstructure of studied samples was determined by SEM and XRD methods. The average grain size varied from 38 to 85 nm, depending on the preparation technique; however, it was also observed that agglomerations consisted of smaller crystallites ca. 10–30 nm. Using the EDX method, it was found that a crucial role in the preparation of nanocrystalline δ-Bi₂O₃ glass-ceramics was played by Si and Al impurities and their glass forming oxides from ceramic crucible. By impedance spectroscopy (IS), the temperature dependencies of electric conductivity (via oxygen ions) were studied and the activation energies of conductivity were determined.

This paper presents results for systems formed in a solid glassy state after nanocrystallization process above the glass temperature. We analyze electric conductivity and relaxation processes after such treatment under high temperature (HT) and high pressure (HP-HT) as well. The latter leads to ca. 8% increase of density, two decades (100) increase of electric conductivity as well as qualitative changes in relaxation processes. The previtreous-type changes of the relaxation time on cooling is analyzed by the use of critical-like and the 'critical-activated' description. Presented results correspond well with obtained for this material and shown in ref. [8]. The evidence for pressure evolution of the glass and crystallization temperatures, indicating the unique possibility of maxima and crossovers is also reported.

In our recent papers, it was shown that the thermal nanocrystallization of glassy analogs of selected cathode materials led to a substantial increase in electrical conductivity. The advantage of this technique is the lack of carbon additive during synthesis. In this paper, the electrochemical performance of nanocrystalline LiFePO₄ (LFP) and LiFe_0.88V_0.08PO₄ (LFVP) cathode materials was studied and compared with commercially purchased high-performance LiFePO₄ (C-LFP). The structure of the nanocrystalline materials was confirmed using X-ray diffractometry. The laboratory cells were tested at a wide variety of loads ranging from 0.1 to 3 C-rate. Their performance is discussed with reference to their microstructure and electrical conductivity. LFP exhibited a modest electrochemical performance, while the gravimetric capacity of LFVP reached ca. 100 mAh/g. This value is lower than the theoretical capacity, probably due to the residual glassy matrix in which the nanocrystallites are embedded, and thus does not play a significant role in the electrochemistry of the material. The relative capacity fade at high loads was, however, comparable to that of the commercially purchased high-performance LFP. Further optimization of the crystallites-to-matrix ratio could possibly result in further improvement of the electrochemical performance of nanocrystallized LFVP glasses.