Lawrence Berkeley National Laboratory

Engineered oxygen vacancies in Mn-rich DRX oxyfluorides promote the DRX-to-δ transformation by enabling tetrahedral (Td) site activation. A 50 °C CCCV protocol activates δ phase within one cycle, and Ti/Al co-doping improves cycling stability. 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 their practical implementation. Here, we synthesized a fluorine-containing, d 0 transition-metal (TM)-free, Mn-rich DRX with oxygen vacancies (Li 1.1 Mn 0.9 O 1.8− z F 0.2 , OV-M90) and investigated the roles of lithium and oxygen non-stoichiometry, redox reactions, and Mn migration in the DRX-to-δ transformation. We found that the 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 d 0 TM dopants slow the transformation, they substantially improve the cycling stability. The Ti and Al co-doped Li 1.05 Mn 0.85 Ti 0.05 Al 0.05 O 1.8− z F 0.1 (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 the kinetic limitations while improving the cycling stability, paving the way for the development of cost-effective, high-energy cathodes for next-generation lithium-ion batteries.