First-Principles Investigation on MgMn₂O_4–yF_y: Fluorine Doping as a Strategy for Magnesium-Ion Batteries beyond the Usual Cation Substitution

High Resolution Image Download MS PowerPoint Slide Magnesium-ion batteries (MIBs) can be competitive against lithium-ion batteries, and the tetragonal spinel-type compound MgMn 2 O 4 is very promising as an electrode-active material. The high voltage delivered by manganese is a significant advantage of these materials, while sluggish kinetics and structural instability during charge/discharge cycling are the main disadvantages. The Jahn–Teller effect induced by Mn(III) can contribute to the structural changes and degradation of the electrode. Doping and element substitution are strategies to improve properties, including replacing manganese with other metals and oxygen with another anion. Anion substitution has been less explored compared to cation substitution. Calculations based on Density Functional Theory (DFT) are employed here to study the effect of the substitution of oxygen atoms by fluorine in the tetragonal spinel. It is found that after F-doping, the cooperative distortion of the unit cell due to the Jahn–Teller effect of Mn(III) can be suppressed for certain distributions of ions, opening a way to improve the properties of the electrode materials for MIBs. According to the DFT results, a transition from tetragonally distorted Mg 8 Mn 16 O 32 to nondistorted (cubic) Mg 8 Mn 16 O 28 F 4 is predicted. A particular distribution of atoms for the composition Mg 6 Mn 12 O 21 F 3, which changes the crystal symmetry, is discussed. In the quest for improved electrodes for MIBs and to compare theoretical calculations with experimental results, we have synthesized MgMn 2 O 4 – y F y . The preliminary experimental results are encouraging. The theoretical and experimental results suggest that F-doping can enhance the structural stability of the spinel during (de)magnesiation. More efforts should be made to design anion-doped spinel and optimize its synthesis and properties.