Variation of the chemical bonding nature of LiMn_2-xNi_xO₄ spinel oxides upon delithiation and lithiation reactions

The effect of nickel substitution on the chemical bonding nature of spinel lithium manganate has been investigated by performing comparative X-ray absorption spectroscopic analyses for Ni- and Cr-substituted LiMn_2-xM_x O₄ (M = Ni and Cr) compounds. The Mn K-edge X-ray absorption spectroscopic results presented here clarify that the replacement of Mn with Ni or Cr gives rise not only to the increase of the Mn oxidation state but also to the suppression of the tetragonal distortion in cubic spinel lattice. From the extended X-ray absorption fine structure analyses at the Ni and Cr K-edges, it has been confirmed that the substitution of nickel ion induces lower structural stability of the spinel lattice compared to that of chromium ion. Especially for the lithiated spinel phase, a remarkable tetragonal distortion is observed for the Ni-substituted case in contrast to the Cr-substituted one. On the basis of the present experimental findings, it is concluded that the substitution of Mn with Ni is less effective in stabilizing the cubic spinel lattice than that with Cr, which is responsible for the inferior electrochemical performance of LiMn_2-xNuO₄.