Reversible anionic redox reactions represent a transformational change for creating advanced highenergy-density positive electrode materials for Li-ion batteries. Their activation mechanism is frequently linked to ligand-to-metal charge transfer (LMCT) processes, which have not been yet fully validated experimentally due to the lack of suitable model materials. Here, we show that the activation of anionic redox in cation-disordered rock-salt Li1.17Ti0.58Ni0.25O2 involves a long-lived intermediate Ni 3+/4+ species, which can fully evolve to Ni 2+ during relaxation. Combining electrochemical analysis and spectroscopic techniques, we quantitatively identified that the reduction of such Ni 3+/4+ goes through a dynamic LMCT process (Ni 3+/4+ -O 2− → Ni 2+ -O n− ). Besides providing the first experimental validation of previous theoretical hypothesis, our finding also helps to rationalize several unusual peculiarities associated with anionic redox, such as cationic-anionic redox inversion and voltage hysteresis. Altogether, this work provides additional guidance for designing high-capacity electrodes by screening cationic species in appropriately mediating ligand-to-metal charge transfer.

All-solid-state batteries (ASSBs) are the subject of large enthusiasm as they could boost by 50% the energy density of today's Li-ion batteries provided that several fundamental/practical roadblocks can be solved.Focusing on the interface between the solid electrolyte and cathode active material (CAM), we herein studied the chemical/electrochemical compatibility of coated-layered oxide LiNi0.6Mn0.2Co0.2O2 with the three main inorganic electrolytes contenders, these being lithium thiophosphate (β-Li3PS4), argyrodite (Li6PS5Cl), and halide (Li3InCl6). Such electrolytes were prepared by either solvent-free or solution chemistry and paired with the CAM to form a composite which was further tested by assembling solid-state batteries using Li0.5In as negative electrode. Amongst the electrolytes prepared by dry routes, the best performing was found to be Li6PS5Cl followed by β-Li3PS4 and lastly Li3InCl6. In contrast, no general trend of benefit or detriment was observed when switching to solution route as it resulted in either performance improvement or deterioration depending on the electrolyte. Additionally, we show a strong dependence of the battery performance upon the presence of carbon additives. Lastly, we unraveled a pronounced chemical/electrochemical incompatibility of Li3InCl6 towards Li6PS5Cl and β-Li3PS4, hence questioning the design of hetero-structural cell architectures. Altogether, we hope these findings to provide guidance in the proper pairing of electrode-electrolytes components for designing highly performing solid-state batteries.