EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

High-entropy oxides (HEOx) are novel materials, which have great application potential due to their unique features coming from entropic stabilization: enhanced structural stability, significantly increased solubility limits and, most of all, the presence of synergistic effects. Although known only since 2015, they have already been proven to exhibit numerous extraordinary properties yielding advantages in many applications including usage in catalysis, as thermal barriers, in solid oxide fuel cells, and promisingly, in Li-ion batteries. In the literature it was proposed that the Li-doped rocksalt-structured HEOx are pure Li-conductors, with the conductivity values exceeding 10^-3 Scm^-1 at room temperature for (Co,Cu,Mg,Ni,Zn)0.7Li0.3O composition. This feature immediately placed them among the best solid electrolytes for all-solid-state batteries. However, the proposed mechanism of Li conduction occurring through percolating channels created by oxygen vacancies can be considered rather unconvincing, mainly due to the presence of high coulombic repulsion. Thus, elucidation of the true nature of conduction in the Li-doped HEOx was the main goal of our studies.
The high-entropy (Co,Cu,Mg,Ni,Zn)1-xLixO (x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30) oxides were synthesized via the solid-state route with subsequent air-quenching. All of the samples were confirmed to be single-phase, with Fm-3m rocksalt-type structure. The electric properties as a function of Li content in a series were investigated through impedance spectroscopy and DC polarization techniques. It was proven that these materials are in fact mixed ionic-electronic conductors, not as previously believed, the pure ionic ones. This behavior was further supported by electrochemical measurements, showing good performance in Li-ion cells. Additionally, the model of mixed ionic-electronic conduction was postulated in this work.