ABO_3-δ perovskite-type oxides are known for their compositional flexibility, possible formation of various orderings in the cationic and anionic sublattices, and a broad range of resulting physicochemical properties. Those properties can be tuned for a particular application, including usage in reversible Solid Oxide Cells (SOCs), as well as for the oxygen storage in pressure swing-type processes.
This work summarizes general guidelines for designing effectively-working perovskite-type oxygen electrodes in SOCs, as well as presents possibility of obtaining oxygen storage materials (OSMs) with the high capacity and low operation temperature. In particular, the A-site layered RE(Ba,Sr)Co_2-yMn_yO_5+δ (RE: selected rare-earth cations; 0 ≤ y ≤ 2) oxides are discussed in more details, as it can be shown that the properly selected Mn substitution results in the high electrocatalytic activity toward the oxygen reduction reaction (ORR), while different Mn content is preferred for the oxygen storage processes [1, 2]. Less commonly studied substitution with Cu is also shown as the effective way of altering physicochemical characteristics, and allows designing electrocatalytically-active RE(Ba,Sr)Co_2-yCu_yO_5+δ series [3]. Furthermore, Co-free La_1-x(Ba,Sr)_xCuO_3-δcompositions can be also designed and obtained, showing promising performance when used as the SOC oxygen electrodes [4, 5]. Notably, the recently emerging high entropy approach provides unique new opportunities, as the resulting multicomponent perovskites may exhibit properties crossing the commonly observed rule of mixtures [6].

Low carbon emissions are perceived as the main target and direction in the global development. It is therefore of importance to explore new energy conversion technologies, to efficiently take advantage of the available green energy resources. Solid Oxide Cells (SOCs) are considered as one of the most promising options, since depending on the demand, they can provide both hydrogen and electricity generation in the same device. In SOCs, the main decisive factor for the efficiency is performance of the oxygen electrode, of which cobalt-containing perovskite-type materials are usually utilized due to their extraordinary electrocatalytic properties at lowered operation temperatures (500-800 °C) [1]. Meanwhile, given that the new concept of the high entropy oxides is proved to be very successful in materials science, it is of great interest to develop and study novel, multicomponent perovskites as candidate oxygen electrode materials. In fact, initial data showed promising performance, with a possibility to limit Co content [2]. Apart from the chemical content optimization, morphology of the oxygen electrode can be also enhanced, which is undoubtedly crucial to influence the oxygen reduction/evolution reaction mechanism. This can be potentially realized by the electrospinning technique, bringing new possibilities for improving the oxygen electrodes.
Taking all those concerns mentioned above into account, in this work, perovskite-type materials with varied substitution, La_0.6Sr_0.4Ni_0.15Mn_0.15Fe_0.15Cu_yCo_0.55-yO_3-δ (y = 0.05-0.20) were synthesized and characterized systematically. X-ray diffraction results confirmed that all compounds are well-crystallized, without any impurities observed, and exhibit rhombohedral symmetry (R-3c). Their structures are stable at high temperatures, up to 900 °C. Only slight variations of the oxygen content with temperature were measured, suggesting mild thermal expansion behavior. High total electrical conductivity was observed for all compounds, above 200 S cm^-1, and interestingly, a negative Seebeck coefficient was detected, suggesting that the main charge carriers are electrons (polarons). The electrochemical characterization in symmetrical cells (based on GDC solid electrolyte) showed an increased catalytic activity with the increasing Co content. However, even the relatively low Co content La_0.6Sr_0.4Ni_0.15Mn_0.15Fe_0.15Cu_0.20Co_0.35O_3-δ electrode demonstrated a desired, low polarization resistance value of only 0.018 Ω cm² at 850 °C. This could be further enhanced by over 20%, if the material was obtained by the electrospinning. The excellent performance was also proved in the full cell measurements, in which a peak power density over 1.0 W cm^-2 was reached at 850 °C, as well as a promising performance was measured in the electrolysis mode.

Today, Li-ion batteries play a vital role as energy storage devices, dominating both, the market for portable electronic products, and the electric vehicles (EV) sector. The burgeoning EV market is pushing for greater cruising ranges, which necessitates the development of novel, high-energy density cathodes and anodes. In terms of cathode materials, there are currently two noteworthy approaches. One involves further development of Ni-rich layered oxides, with Ni content above 0.9 [1], while the other one focuses on Li- and Mn-rich oxides, which possess specific structural properties [2]. Both groups of materials hold promise for significant advancements in constructing high-capacity and high-power density cells, thanks to their very high reversible discharge capacity (> 200 mAh g^-1), high operating voltage (~3.7 V vs. Li/Li⁺), and relatively low costs. However, these oxides still face severe issues, including surface sensitivity, structural problems such as Li/Ni mixing effects, and inadequate thermal stability, which limit their practical application. Of particular concern is the presence of lithium residuals like LiOH/Li₂CO₃ in the active material, stemming from the synthesis process. Concerning the anode, a particularly interesting direction is combining conversion and alloying reaction mechanisms within a single compound (so called conversion-alloying materials, CAMs) [3]. However, CAMs still suffer from insufficient cycling stability and the only solution proposed in the literature so far is to employ advanced synthesis methods and additives, which are often expensive and difficult to scale. Conversely, the recently discovered high-entropy oxides (HEOs) show excellent cyclability when used as anodes in Li-ion cells, regardless of the synthesis method and resulting particle size.

Combining the above concepts, in this study we synthesized and systematically characterized selected Ni-rich LiNi_0.905Co_0.043Al_0.052O₂ (NCA905) and Li- and Mn-rich Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ (NMC135413) cathode materials. Cathode layers were then obtained, with high active material loadings of ca. 6 mg cm^-2. Both types of cathodes were assembled initially alongside standard graphite anode, and also with the novel high-entropy Sn_0.8(Co_0.2Mg_0.2Mn_0.2Ni_0.2Zn_0.2)_2.2O₄-based anode. Various issues related to combining those electrodes into full cells were studied, including the selection of negative/positive ratio, electrode prelithiation process, and electrolyte additives. The resultant optimized full cells exhibited very good electrochemical performance. For example, the NMC135413@Graphite anode full cell delivered an initial discharge capacity of more than 186 mAh g^-1 at 0.5 C current density (cathode limited), and a very high energy density of 370 Wh kg^-1. A capacity retention of 80% was measured after 400 cycles, indicating very promising electrochemical characteristics.