Study of Lithium-Excess Manganese Oxyfluorides with Mn²⁺/Mn³⁺ Ions for Li Storage Applications

To further increase energy density of rechargeable Li batteries, the development of high-capacity positive electrode materials is required. Recently, our group has reported that the use of anionic redox in Li-excess Mn-based oxides as high-capacity positive electrode materials. ¹⁾In this study, this concept is extended to Li-excess Mn-based oxyfluorides, in which higher operating voltage is expected for anionic redox associated with the incorporation of fluoride ions with higher electronegativity. ²⁾<br/>Synthesis of oxyfluoride with different compositions has been conducted on the basis of a triangular phase diagram consisting of LiMnOF, Li₂MnO₂F, and Li₂MnOF₂. Single-phase samples are obtained for Li₂MnO₂F with Mn³⁺ ions and Li₂Mn²⁺O_3/2F_3/2 with Mn²⁺:Mn³⁺ = 1:1, synthesized by mechanical milling. These samples are crystallized into the cation-/anion-disordered rocksalt-type structure with low crystallinity. On the other hand, a single phase sample cannot be obtained for Li₂MnOF₂ with Mn²⁺ ions. Although Li₂MnOF₂ was electrochemically inactive, Li₂MnO₂F and Li₂Mn²⁺O_3/2F_3/2 deliver a large reversible capacities of &gt;250 mA h g^–1. Note that the capacity retention is partly improved by the enrichment of Mn²⁺ ions with less anionic redox. Based on these results, we will further discuss the possibility of Li-excess manganese-based oxyfluorides with different chemical compositions for Li storage applications.<br/><br/>References<br/>1) N. Yabuuchi et al., Nature Commun., 7, 13814 (2016).<br/>2) N. Yabuuchi, Chemical Record, 19, 690 (2019).