Novel materials can considerably improve storage capacity and cycling stability of rechargeable batteries. Among these materials are high-entropy oxides (HEO), the stability of which results from a disordered distribution of the elements. With HEO, electrochemical properties can be tailored, as was found by scientists at Karlsruhe Institute of Technology (KIT).

An open-access paper on their work is published in the journal Nature Communications.

HEOs incorporate multiple metal cations into single-phase crystal structures; interactions among the various metal cations lead to interesting novel and unexpected properties.

Recently, a new class of oxide systems, also known as high entropy oxides (HEO), was formulated and reported with first demonstrations for transition-metal-based HEO (TM-HEO), rare-earth-based HEO (RE-HEO) and mixed HEO (TM-RE-HEO). HEO are based on a new, quite revolutionary concept of entropy stabilization, that is, to stabilize a certain crystal structure that can differ from the typical crystal structures of the constituent elements, thereby increasing the configurational entropy of the resulting compounds. This concept was first reported for metallic high entropy alloys (HEA). In recent years, the study of HEA has grown into an independent field of materials research, as evidenced by numerous publications10.

Several reports on TM-HEO, RE-HEO, and mixed TM-RE-HEO have demonstrated that high entropy stabilization in oxides with 5 or more cations in equiatomic concentrations leads to the formation of single-phase rock-salt, fluorite, or perovskite structures. These compounds often show interesting and unexpected properties, such as extraordinarily high room temperature Li-ion conductivities for solid state electrolytes in TM-HEO, very narrow and tailored band gaps in RE-HEO and colossal dielectric constants in TM-HEO. The main driver for the growing interest in HEO is the potential to obtain novel properties by exploiting the enormous number of possible elemental combinations.

—Sarkar et al.

Top images: The active material studied with high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDX). Bottom: Schematics of the proposed de-/lithiation mechanism during the conversion reaction of TM-HEO. Sarkar et al.

Among the most important properties of batteries are their storage capacity and their cycling stability, i.e. the number of possible charging and discharging processes without any loss of capacity. Due to its high stability, an entirely new class of materials called high-entropy oxides (HEO) is expected to result in major improvements. Moreover, electrochemical properties of HEO can be customized by varying their compositions.

Scientists of KIT’s Institute of Nanotechnology (INT) and Karlsruhe Nano Micro Facility (KNMF), of the Helmholtz Institute Ulm (HIU) established jointly by KIT and Ulm University, and of the Indian Institute of Technology in Madras have now demonstrated the suitability of HEO as conversion materials for reversible lithium storage.

Conversion batteries based on electrochemical material conversion allow for an increase of the stored amount of energy, while battery weight is reduced. The scientists used HEO to produce conversion-based electrodes that survived more than 500 charging cycles without any significant degradation of capacity.

The Nanostructured Materials group of Professor Horst Hahn, Director of KIT’s INT, is among the pioneers of research into high-entropy oxides. The scientists published several of few publications about these novel materials. The special properties of HEO result from entropy stabilization. This makes them comparable to the already better known high-entropy alloys.

Entropy-stabilized HEO are complex oxides containing five or more different metal cations of the same amounts and exhibiting a single-phase crystal structure. Although typical crystal structures of the elements differ considerably, they form a joint lattice and distribute to the positions in the crystal without any apparent order. This disorder, also referred to as high entropy, stabilizes the material, probably because it impairs migration of defects in the lattice.

Thanks to the high stability, the interactions of the different metal cations, and the high number of feasible element combinations, HEO open up undreamt-of opportunities.

—Professor Horst Hahn

The study in Nature Communications concentrated on HEOs based on transition metals (TM-HEOs), which are characterized by high lithium ion conductivity. By means of transmission electron microscopy (TEM), the researchers studied the structure of TM-HEO and its impact on the conversion reaction.

They found that removal of one element only reduces entropy and adversely affects cycling stability. Each individual element influences the electrochemical behavior of the TM-HEO, such that the materials can be tailored to various applications.

The result is a modular approach to the systematic development of electrode materials.

In this study, to our knowledge for the first time, it is shown that high entropy oxides are very promising materials for reversible electrochemical energy storage. The variation of the composition of the oxides allows tailoring the Li-storage properties of the active material. The incorporation of different elements into HEO offers a modular approach for the systematic design of the electrode material. Additionally, it is shown that entropy-stabilized oxides have high capacity retention and exhibit a de-/lithiation behavior, which is drastically different from classical conversion materials.

The new effect is attributed to configurational entropy stabilization of the lattice, which conserves the original rock-salt structure while serving as a permanent host matrix for the conversion cycles. Based on these—necessarily limited—first, but promising results, further investigations toward high entropy oxide electrode materials should be pursued to explore their full potential for energy storage applications.

—Sarkar et al.

Resources

• Abhishek Sarkar, Leonardo Velasco, Di Wang, Qingsong Wang, Gopichand Talasila, Lea de Biasi, Christian Kübel, Torsten Brezesinski, Subramshu S. Bhattacharya, Horst Hahn & Ben Breitung (2018) “High entropy oxides for reversible energy storage.” Nature Communications doi: 10.1038/s41467-018-05774-5