Durable Manganese-Based Li-Excess Electrode Material without Voltage Decay: Metastable and Nanosized Li₂MnO_1.5F_1.5

High Resolution Image Download MS PowerPoint Slide Li-excess manganese-based oxides have been proposed as high-capacity positive electrode materials, but voltage decay associated with gradual oxygen loss hinders its use for practical applications. Herein, Li-excess manganese oxides with different fluorine contents are synthesized by high-energy mechanical milling. Although Li 2 MnOF 2 with only divalent manganese ions cannot be synthesized, Li 2 MnO 2 F and Li 2 MnO 1.5 F 1.5 are successfully synthesized. When the samples are charged to 5.0 V, both oxyfluorides deliver large reversible capacities, ∼350 mA h g –1 and ∼1000 mWh g –1 . However, insufficient capacity retention is also observed because of the instability of anionic redox. In contrast, better capacity retention and higher energy density (730 mWh g –1 ) are obtained for Li 2 MnO 1.5 F 1.5 with a 4.4 V cutoff because of the enrichment of fluoride ions and activation of Mn 2+ /Mn 4+ cationic redox. Moreover, electrode durability is significantly improved by using a highly concentrated electrolyte, and good capacity retention without voltage decay is achieved for >180 cycles.