PNNL

Abstract

The capacity of lithium transition-metal (TM) oxide cathodes is directly linked to the size of charge reservoir(s) residing in cationic redox of TM and/or anionic redox of oxygen, which traditionally decreases with cycling as a result of chemical, structural or mechanical/morphological fatigue. In the present study, we show that capacity increase over 125 % can be achieved upon cycling of a Mn-rich cation-disordered rocksalt oxyfluoride (F-DRX) cathode, Li1.2Mn0.7Nb0.1O1.8F0.2 (F10), leading to exceptional energy density even after extended cycling. Our study reveals that in Mn- and F-rich DRX environments, continuous cycling with Mn3+/Mn4+ redox accompanied by a small degree of O redox participation can lead to local structural rearrangement and the growth of transformed domains with spinel-like features, characterized by the increasing presence of redox activities near 3 V on discharge and 4 V on charge. The effective integration of the locally transformed phase within the cubic rocksalt framework promotes cycling stability and improves Li transport kinetics. Our study provides important design insights on how to further exploit Mn-redox chemistry for developing long-lasting high-energy cathode materials.