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.