Study of local disorder in LiMn(Cr,Ni)O₂ compounds by extended X-ray absorption fine structure measurements

We have studied local structure of LiMnO₂, LiMn _0.65Cr_0.35O₂ and LiMn_0.5Ni _0.5O₂ compounds by Mn K-edge extended X-ray absorption fine structure measurements. The local structure of LiMnO₂ is found to be consistent with Jahn-Teller distorted MnO₆ octahedra characterized by two different Mn-O bond distances. The Jahn-Teller distortions are suppressed in the Cr and Ni substituted compounds, resulting a single Mn-O distance. However, the Cr atoms tend to occupy a site at a longer distance from Mn in the host lattice (Mn-Cr distance is longer than Mn-Mn distance), unlike the Ni atoms which prefer a site closer to the Mn atoms (Mn-Ni distance is shorter than Mn-Mn distance). Incidentally, Mn-O and Mn-Mn bonds are substantially stiffer in the Cr and Ni substituted compounds. In addition, the static atomic disorder is confined around Cr atoms in the LiMn _0.65Cr_0.35O₂, that is different from the case of LiMn_0.5Ni_0.5O₂ in which larger static disorder appears in the proximity of the Mn atoms. The results suggest that the differences in the local structure of different compounds should be the likely reason for their differing battery characteristics.