Capturing dynamic ligand-to-metal charge transfer with a long-lived cationic intermediate for anionic redox

Abstract

Reversible anionic redox reactions represent a transformational change for creating advanced high-energy-density positive-electrode materials for lithium-ion batteries. The activation mechanism of these reactions is frequently linked to ligand-to-metal charge transfer (LMCT) processes, which have not been 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 Li_1.17Ti_0.58Ni_0.25O₂ involves a long-lived intermediate Ni^3+/4+ species, which can fully evolve to Ni²⁺ during relaxation. Combining electrochemical analysis and spectroscopic techniques, we quantitatively identified that the reduction of this Ni^3+/4+ species goes through a dynamic LMCT process (Ni^3+/4+-O^2- → Ni²⁺-O^n-). Our findings provide experimental validation of previous theoretical hypotheses and help to rationalize several peculiarities associated with anionic redox, such as cationic-anionic redox inversion and voltage hysteresis. This work also provides additional guidance for designing high-capacity electrodes by screening appropriate cationic species for mediating LMCT.