Understanding and optimizing the « anionic redox » thanks to the theory

The study of the “anionic redox” process has enthused for several years the scientific community of the batteries field. Mastering the phenomenon could permit to increase the energy density of the storage systems and to improve batetry lifetime. A team of RS2E researchers, from the ICGM and Collège de France, used theoretical tools to understand the mechanisms and the limitations of the anionic redox process and to optimize its use in order to improve energy densities.

Increased load capacity against irreversibility in discharge: That is the challenge of the anionic redox process, scrutinized by many scientists. Indeed, if the anionic redox could be efficiently combined with cationic redox, we could increase the lifetime of the batteries by 20%, doubling the charges capacity stored in the positive electrode (cf. article)

Unfortunately, a destabilization of the material structure usually follows the anionic redox process. That leads to irreversibility of the first charge/discharge cycle, a decrease in capacitance and potential, cationic migration and oxygen release.

A team of researchers, led by Marie-Liesse Doublet from the ICGM, has tried to understand the origin of this irreversibility and to propose solutions to improve the performances of the electrodes. The scientists have used conceptual and computational tools from theoretical chemistry to identify chemical and electronic descriptors that control the reversibility of electrochemical reactions involvinganions. Theirs detailed conclusions can be found in an article published May 18 in the journal Nature Materials.

Make concessions

In order to limit or suppress the irreversibility due to the anionic redox in transition metal oxides enriched in alkali (Li or Na), previous studies proposed to increase the covalence of the metal-oxygen bonds, to replace the transition metal by an electrochemically inactive element or to create a cationic disorder.

Contrary to these assertions, the study conducted by the RS2E team shew that determining parameter of the reversibility of anionic redox is the number of holes by oxygen generated in charge. If this number remains below the limit value of 1/3, the anionic redox is predicted to be totally reversible, without O2 release.

Other conclusion, it is advised to use tridimensional electrodes, not two-dimensional ones, in order to avoid the well-known structural instabilities of the lamellar structures with low Li or Na content.

Unfortunately, the authors of the article point out that there are few materials that meet all the criteria, while being cheap, non-toxic and based on abundant elements. In addition, it remains to find the ideal composition to exploit the anionic redox at the industrial level.

Between high potential, high capacity and structural stability, it will be necessary to choose…

References:

Unified picture of anionic redox in Li/Na-ion batteries

Mouna Ben Yahia, Jean Vergnet, Matthieu Saubanère, Marie-Liesse Doublet

Nature Materials, 18/03/19, DOI : 10.1038/s41563-019-0318-3

Contacts: Marie-Liesse Doublet, marie-liesse.doublet@umontpellier.fr / Matthieu Saubanère, matthieu.saubanere@umontpellier.fr