High entropy oxides (HEOs) are a recently discovered class of materials where a particular crystal structure, which in general is different from that of the parent compounds, is stabilized in a multicomponent system, characterized by a large amount of configurational entropy [1,2]. In the recent years, these materials are gathering increasing attention for many technologically relevant applications. In particular, the main driver for the growing interest in HEOs is the potential to obtain novel properties by exploiting the enormous number of possible elemental combinations.
The high complexity of these systems requires advanced characterization tools for understanding the reaction mechanisms in which they are involved. Here we report two scientific cases, where operando and ex situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) are employed to investigate (Cu,Mg,Ni,Co,Zn)O as anode and Li(Fe_xCr_xMn_2-2x-yTi_yO₄) as cathode for Li-ion batteries. XAS proved to be particularly suitable for these systems, where element selectivity is mandatory to discern the role of each cation. Moreover, multivariate curve resolution (MCR) is here presented as a novel strategy to treat operando XAS data, with the aim of getting detailed information on the number, nature and concentration evolution of the species as a function of the stored charge [3].
(Ni_0.2Co_0.2Mg_0.2Zn_0.2Cu_0.2)O was investigated through operando XAS with the aim of elucidating the working mechanism during the first cycle [4]. A complex redox mechanism was revealed, developing through the reduction of the transition metals, which triggers the conversion reaction below 1 V. The conversion is irreversible and incomplete, leading to the final collapse of the halite structure. However, the structure is retained up to the 60% of charge before collapsing, thus proving the beneficial role of the configuration entropy [5].
Li(Fe_xCr_xMn_2-2x-yTi_yO₄) was tested instead as cathode. Compared to prototype LiMn₂O₄, the substitution of Mn with Fe, Cr and Ti suppresses the Jahn-Teller distortion, increasing the cycling voltage range (4.8-1.5 V), leading to higher capacity and significantly improved stability. XAS demonstrated that the only redox-active metal is Mn, while Fe, Cr and Ti are electrochemically inactive. Operando XRD, in turn, gave evidence that the spinel structure is maintained throughout the lithiation/delithiation process, proving that no phase segregation or Jahn-Teller distortion ever occurs. Curiously, in the same spinel material without Cr phase segregation was promptly observed during lithiation [5]; this demonstrates that the combination of multi-elements may indeed be beneficial and lead to unexpected properties.
 
[1] C. Rost et al. Nat. Commun., (2015), 6 8485
[2] M. Fracchia et al.  Nat. Commun., (2022), 13, 2977
[3] F. Tavani et al. Phys. Chem. Chem. Phys., (2021), 23, 26575
[4] P. Ghigna et al. ACS Appl. Mater. Interfaces (2020), 12, 50344-50354
[5] D. Callegari et al. J. Mater. Chem. C, (2022), 10, 8994