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 Mn^III (t_2g ³–e_g ¹). 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.