Durable Manganese-Based Li-Excess Electrode Material without Voltage Decay: Metastable and Nanosized Li2MnO1.5F1.5

Abstract

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 Li2MnOF2 with only divalent manganese ions cannot be synthesized, Li2MnO2F and Li2MnO1.5F1.5 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 Li2MnO1.5F1.5 with a 4.4 V cutoff because of the enrichment of fluoride ions and activation of Mn2+/Mn4+ 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.