A class of compositions in the Li—Mn—O—F chemical space for Li-ion cathode materials. The compositions are cobalt-free, high-capacity Li-ion battery cathode materials synthesized with cation-disordered rocksalt (DRX) oxide or oxyfluorides, with the general formula LixMn2-xO2-yFy (1.1≤x≤1.3333; 0≤y≤0.6667). The compositions are characterized by: (i) high capacities (e.g., >240 mAh/g); (ii) high energy densities (e.g., >750 Wh/kg between 1.5-4.8V); (iii) favorable cyclability; and (iv) low cost.