Vol 59, No 3 (2023)

Recent experimental results of the dependence on oxygen partial pressure, P(O2), of the small polarons concentration in the monomolecular surface layer of doped ceria, are interpreted theoretically. Two different, experimental conditions are examined. First, the highly reduced surface of Sm0.2Ce0.8O1.9 – δ (SDC) is treated and the [Ce3+]s--P(O2) relations are evaluated. The experimental data are explained by a metallic surface layer, approximately neutral, i.e. with negligible transfer of charge to the bulk. The surface is metallic as the outer electrons on Ce3+ ions, which are small polarons when under low concentration, lose their localization under high concentration. Second, the surface of Pr0.1Ce0.9O2 – δ (PCO) is examined. The experimental data of [Pr3+]s--P(O2) relations are explained by a semiconducting, negatively charged, monomolecular surface layer forming a double layer with a positive bulk. In both SDC and PCO the surface defect concentration dependence on P(O2) is different from that of the bulk.

The role of primary (the particles’ size) and secondary (the particles’ content in the material) size effects of metal–ion-exchanger composites in the oxygen electrochemical reduction is elucidated. To this purpose, metal–ion-exchanger particulate composites with different grain size and the metal (Cu) particles’ content are prepared as spherical grains, based on macroporous sulfonic cation-exchange matrix (Lewatit K 2620). X-ray diffraction analysis showed the deposited metal basic particles to be nanosized. A special feature of the metal particles is that during the repeated cycles of their chemical deposition into the ion-exchange matrix pores both the capacity ε, and the particles’ radius r0 increased. On this reason, the primary and secondary size effects appeared being interconnected in a common nanosized complex f=ε/r0. With the increasing of the capacity the complex increased up to certain limiting value, which is connected with percolation transition from separate metal clusters to collective associates. Correspondingly, the reduced oxygen specific amount also reached its constant value. The oxygen electroreduction process reached quasi-steady-state regime.

The effect of the nature of the transition metal ion (electron configuration 3d0 and 4d0) on the local structure and electrochemical properties of lithium-rich oxyfluorides with disordered rock-salt structure Li1 + x(MеMn3+)1 – xO2 – yFy, where Mе = Ti4+, Nb5+, 0.2 ≤ x ≤ 0.288 and 0.05 ≤ y ≤ 0.15 is studied. The compounds are thoroughly investigated by the methods of X-ray diffraction analysis, scanning electron microscopy, granulometry, electron spin resonance spectroscopy, and galvanostatic cycling. The galvanostatic-cycling curves of the compounds have two plateaus in the voltage regions of 3.3–3.4 and 4.1–4.3 V. They can be attributed to redox-processes involving two couples: Mn3+/Mn4+ and O2–/O–. In the case of Ti-containing oxyfluorides with disordered rock-salt structure, with the increasing of fluorine content the contribution from O2–/O–-couple during the electrochemical process decreases. In both systems of the oxyfluorides with disordered rock-salt structure we observed formation of paramagnetic clusters Mn3+–O–Mn4+ whose number increased with the increasing of Mn content. The largest clusterization is observed for the sample Li1.266Nb0.217Mn0.55O1.85F0.15. At the same time, the diffusion coefficient for Nb-containing oxyfluorides with disordered rock-salt structure is lower by order of magnitude than for the Ti-containing ones. This may be connected with the strongest clustering of Mn3+ ions, which hinders the Li+ ion macrodiffusion and, as a consequence, deteriorates the kinetics of the process.

The effects of mechanical activation in a planetary mill and the addition of fusible additives on the conduction properties of the Li1.3Al0.3Ti1.7(PO4)3 (LATP) solid electrolyte with the NASICON structure are compared. According to the results of impedance measurements, the mechanical activation increases the total conductivity of this material from 0.57 × 10–4 to 1.20 × 10–4 S cm–1, whereas the introduction of 5 wt % of fusible additives LiPO3 and Li2B4O7 increases the conductivity to 1.53 × 10–4 and 1.50 × 10–4 S cm–1, respectively. The electronic conductivity of samples does not exceed 10–9–10–8 S cm–1. According to the temperature dependence of the conductivity, the LATP sample containing Li2B4O7 (5 wt %) demonstrates the lowest activation energy equal to 0.29 eV.