Atomic-scale observation of phase transition of MgMn2O4 cubic spinel upon the charging in Mg-ion battery

Direct observation of demagnesiated structure of MgMn2O4 spinel oxides at atomic scale has been achieved using spherical aberration-corrected scanning transmission electron microscopy (STEM) with high-angle annular-dark-field (HAADF) and annular-bright-field (ABF) techniques. Upon the Mg ions extraction from MgMn2O4 spinel oxides, structural transition from cubic to tetragonal phases was observed. The phase transition is a result of Mn octahedral distortion due to cooperative Jahn–Teller distortion of six-coordinate MnIII (t2g3–eg1). On the basis of HAADF/ABF micrographs and electron energy-loss spectrometer (EELS), it is suggested that Mn3+/Mn4+ and/or Mn3+/Mn5+ redox couples play a crucial role in the first electrochemical Mg deintercalation. This is the first report on observation of cubic-tetragonal phase transition in charged spinel MgMn2O4 which shed new insight into the magnesium storage mechanism in this important cathode material for Mg-ion batteries.