Tailored Valence Chemistry Mitigating Capacity Fading in Mn-Based Disordered Rocksalt Cathodes

Cation-disordered rocksalt (DRX) cathodes are promising alternatives to conventional nickel-/cobalt-based layered oxides, offering high energy density with earth-abundant elements such as manganese (Mn) and titanium (Ti). However, their practical deployment has remained limited by severe capacity fading during extended cycling. In this study, we identify over-reduction of Mn3+ and its subsequent dissolution as a primary driver of performance decay in Li1.2Mn0.5Ti0.3O1.9F0.1, a representative Mn/Ti-based DRX cathode. It is shown that repeated cycles involving oxygen redox lead to an increasing fraction of over-reduced Mn2+ at the end of discharge, resulting in extensive Mn dissolution and structural deterioration. Accordingly, we demonstrate that elevating the overall valence state of Mn by adjusting Mn ratio effectively lessens over-reduction of Mn3+, thereby delivering substantially improved cycling stability. These findings underscore the critical importance of Mn redox-state regulation in the compositional design of Mn-based DRXs for achieving long-term structural and electrochemical stability as high-energy-density cathodes.