A team led by researchers from the Karlsruhe Institute of Technology (KIT) in Germany is proposing a new class of high entropy materials for energy storage applications. In a paper in the RSC journal Energy & Environmental Science, they report on multi-anionic and -cationic compounds prepared by facile mechanochemistry using a recently designed multi-cationic transition-metal-based high entropy oxide as the precursor and LiF or NaCl as the reactant, leading to formation of lithiated or sodiated materials.

The Li-containing entropy-stabilized oxyfluoride (Li_x(Co_0.2Cu_0.2Mg_0.2Ni_0.2Zn_0.2)OF_x) exhibits a working potential of 3.4 V vs. Li⁺/Li, enabling its use as a cathode active material.

Unlike conventional (non-entropy-stabilized) oxyfluorides, the new material shows enhanced Li storage properties due to entropy stabilization, which, in general, facilitates tailoring the cycling performance by varying the constituent elements in yet unprecedented ways.

The concept of entropy stabilization is also applicable to Na-containing oxychlorides with a rock-salt structure, thus paving the way toward development of (next-generation) post-Li battery technologies.

High entropy materials (HEMs) are gaining significant interest due to their novel and often unexpected and unprecedented properties in many different areas of application. HEMs are based on the premise of introducing a high configurational entropy (S_config) to stabilize a single-phase structure. … A large number of such high entropy compounds have been synthesized and reported, including carbides, diborides, nitrides, chalcogenides and oxides, with wide-ranging applications as thermoelectrics, dielectrics and for lithium-ion batteries. The latter HEMs, named high entropy oxides (HEOs), have been reported only since 2015 by the pioneering work of Rost et al.

However, until now, there are no literature reports on HEM compounds with more than one anion. Hence, the stabilizing S_config effect only results from the cations present in the crystal structure, since the contribution from the anion site is zero. For this reason, the preparation of a multi-anionic and multi-cationic single-phase structure, showing clear indications of entropy stabilization, is of great interest, especially considering that the configurational entropy gain would be even larger compared to the transition-metal-based HEO systems.

Herein, we report, to the best of our knowledge for the first time, on multi-anionic and multi-cationic high entropy oxyhalides with application in electrochemical energy storage.

—Wang et al.

The researchers used a multi-cationic transition-metal-based HEO (i.e., only oxygen ions occupy the anion site) as the precursor and introduced additional halide (X) and alkali metal ions to produce a multi-anionic and multi-cationic rock-salt type compound (HEOX).

The introduction of monovalent fluorine into the anion lattice of HEO, occupied by divalent oxygen, is charge compensated by incorporation of monovalent lithium (or sodium) into the cation lattice. Since fluorine and oxygen have similar ionic radii, such substitution can be achieved without inducing significant strain in the single-phase rock-salt structure.

With the incorporation of multiple anions into an entropy-stabilized multi-cationic compound, we were able to show, for the first time, that not only the cations but also the anions can be varied while preserving a single-phase rock-salt structure. These compounds constitute a new class of entropy-stabilized materials, with the anion lattice also contributing to the configurational entropy, resulting in an additional structural stabilization gain. Using this approach, we successfully synthesized an oxyfluoride cathode active material with a rock-salt structure for next-generation Li-ion battery applications.

Notably, the entropy stabilization improved the cycling performance considerably. Additionally, this approach enables the reduction of toxic and costly elements in battery cathodes, without significantly affecting the energy density. Taken together, the concept of multi-anionic and multi-cationic high entropy compounds introduces a new class of energy storage materials with unprecedented properties.

—Wang et al.

Resources

• Wang, Q. et al. (2019) “Multi-anionic and -cationic compounds: New high entropy materials for advanced Li-ion batteries.” Energy & Environmental Science doi: 10.1039/c9ee00368a