Abstract
Great hopes have recently been placed on the emergence of anionic redox : a transformational approach enabling the design of high-capacity positive electrodes. However, some questions have been raised in particular about the basis of the origins of anionic redox and its potential for application. This fifth lecture addressed, via a chemistry-theory synergy, the scientific underpinnings that trigger reversible and stable anionic redox activity. In particular, it highlighted the compositional, dimensional and ordering guidelines for the design of many high-energy-density materials. In addition, he highlighted the benefits of a covalent bond compared with an ionic bond, thus limiting the departure of O2 or other substances.
Anionic redox activity can also be found in disordered rocksalt structures, referred to as " DRX ", which feature capacities of up to 300 mAh/g but wide hysteresis. Although disorder is counter-intuitive to Li diffusion, we have shown, using combined experimental-theoretical approaches, how this is not the case in the presence of phases with excess Li. We distinguished between totally disordered materials and those with short-range cationic order, which is detrimental to Li+ diffusion. High-entropy material design, well known in metallurgy and medicine, was thus introduced to the world of batteries. The principle is based on the use of multi-element configuration entropy to stabilize previously unreported pure multi-element phases at average processing temperatures.
The richness of this new family of high-energy-density materials does not, however, guarantee their direct integration into the next generation of batteries. Energy density is just one of many figures of merit for the applicative aspect of these materials. Unfortunately, many technological hurdles have yet to be overcome. The need to limit the release of O2 at high potential has been highlighted, as has the need to reduce the potential hysteresis between charge and discharge which penalizes their energy efficiency, and their potential drop during long cycling. The approaches currently being considered to remedy these difficulties were also presented. With regard to the potential drop, this consists in stopping cation migration via chemical substitutions, the design of new structures featuring different polyhedron connectivities, or even the design of core-crown particles. In addition to the design of oxygen-lacking materials, the approach of partial substitution of oxygen by fluorine is now favored to avoid the need for oxygen. Synthesis approaches are varied, and are based either on ceramic processes in the presence of LiF, or on low-temperature processes in the presence of XeF2, or on the synthesis of new disordered Li2MnO2Fphases. In conclusion, the issue of potential hysteresis has been addressed from a fundamental point of view, through the design of model compounds.
