High-Energy Manganese-Rich Rocksalt Cathodes with Engineered Oxygen Vacancies

Abstract

Manganese-rich (Mn > 0.6) disordered rocksalt (DRX) cathodes undergo structural transformation into a spinel-like δ-phase upon cycling, resulting in enhanced energy density and cycling stability. This transformation is a gradual process, often requiring tens of cycles, which presents challenges in practical implementation. Here we synthesize a fluorine-containing, d0 transition-metal (TM)-free, Mn-rich DRX with oxygen vacancies (Li1.1Mn0.9O1.8-zF0.2, OV-M90) and investigate the roles of lithium and oxygen non-stoichiometry, redox reactions, and Mn migration in the DRX-to-δ transformation. We find that tetrahedral site activation, critical for δ-phase formation, is promoted by synergistic interactions among these factors. A CCCV protocol at 50 °C enables rapid δ-phase activation within a single cycle. While d⁰ TM dopants slow the transformation, they substantially improve cycling stability. The Ti and Al co-doped Li1.05Mn0.85Ti0.05Al0.05O1.8-zF0.1 (OV-M85T5A5) maintains nearly 100% capacity retention over 100 cycles at 30 mA g⁻¹. These findings provide insights into the DRX-to-δ transformation mechanism and present strategies to overcome kinetic limitations while improving cycling stability, paving the way for the development of cost-effective, high-energy cathodes for next-generation lithium-ion batteries.