Anionic redox in Li-rich and Na-rich Transition Metal Oxides (Arich-TMOs) has emerged as a new paradigm to increase the energy density of rechargeable batteries. Ever since, numerous electrodes delivering anionic extra-capacity beyond the theoretical cationic capacity were reported. Unfortunately, most often the anionic capacity achieved in charge is partly irreversible in discharge. A unified picture of anionic redox in Arich-TMOs is here designed to identify the electronic origin of this irreversibility and to propose new directions to improve the cycling performance of the electrodes. The electron localization function (ELF) is introduced as a holistic tool to unambiguously locate the oxygen lone-pairs in the structure and follow their participation in the redox activity of Arich-TMOs. The charge-transfer gap of transition metal oxides is proposed as the pertinent observable to quantify the amount of extra-capacity achievable in charge and its reversibility in discharge, irrespectively of the material chemical composition. From this generalized approach, we conclude that the reversibility of the anionic capacity is limited to a critical number of O-hole per oxygen, h O ≤ 1/3.

To satisfy the long-awaited need of new lithium-ion battery cathode materials with higher energy density, anionic redox chemistry has emerged as a new paradigm that is responsible for the high capacity in Li-rich layered oxides, for example, in Li1.2Ni0.13Mn0.54Co0.13O2 (Li-rich NMC). However, their marketimplementation has been plagued by certain bottlenecks originating intriguingly from the anionic redox activity itself. To fundamentally understand these bottlenecks (voltage fade, hysteresis and sluggish kinetics), we decided to target the ligand by switching to isostructural Li-rich layered sulfides. Herein, we designed new Li1.33-2y/3Ti0.67-y/3FeyS2 cathodes that enlist sustained reversible capacities of ~245 mAh•g-1 due to cumulated cationic (Fe 2+/3+) and anionic (S 2-/ S n-, n < 2) redox processes. In-depth electrochemical analysis revealed nearly zero irreversible capacity during the initial cycle, very small voltage fade upon long cycling, with low voltage hysteresis and fast kinetics, which contrasts positively with respect to their Li-rich NMC oxide analogues. Our study, further complemented with DFT calculations, demonstrates that moving from oxygen to sulfur as the ligand is an adequate strategy to partially mitigate the practical bottlenecks affecting anionic redox, although with an expected penalty in cell voltage. Altogether the present findings provide chemical clues on improving the holistic performance of anionic redox electrodes via ligand tuning, and hence strengthen the feasibility to ultimately capitalize on the energy benefits of oxygen redox.