CaFe₂O₄-Type Post-Spinel Host Compounds as Mg Battery Cathodes

The development of an Mg battery promises a high energy density at a low material cost. A primary roadblock for Mg batteries is the identification of a cathode material with sufficient kinetics and stability. Previous first-principles calculations have suggested a low migration barrier for Mg²⁺ in host compounds with the CaFe₂O₄ (CF) structure. In this work we experimentally investigate the electrochemical properties against Mg metal of several lithium post-spinel compounds with the CF structure including LiVTiO₄, LiCrTiO₄, LiMnSnO₄, and LiFe_0.5Ti_1.5O₄. To open the tunnels for Mg intercalation, the lithium post-spinels are chemically delithiated with Br₂/acetonitrile prior to electrochemical characterization. Structural characterization of the delithiated cathode materials is carried out including powder x-ray diffraction, scanning electron microscopy, and solid-state nuclear magnetic resonance spectroscopy. The capacity, potential, and stability of the cathode materials are detailed along with a consideration of the interactions of the Mg anode/cathode with the Mg(TFSI)₂-based electrolyte. From these results and a comparison of the Mg cathode performance with the performance of the lithium post-spinels as lithium battery cathodes, we further our understanding of the CF structure as a monovalent and multivalent intercalation host.