Study of Average and Local Structures of Variedly Ordered LiNi0.5Mn1.5O4 By Neutron Diffraction

The spinel-type LiNi_0.5Mn_1.5O₄ cathode materials exhibit varied Ni/Mn cation ordering by the synthesis process and subsequent thermal treatment, resulting in differential electrochemical behaviors in Li-ion batteries. In order to achieve a well-defined ordering structure and its influence to the electrochemical properties, a series of LiNi_0.5Mn_1.5O₄ samples are prepared with systematically controlled Ni/Mn ordering by the variant of annealing time at 700 C. The average crystal structures and local structures are characterized by measuring the neutron diffraction patterns and the pair distribution functions (PDF), respectively. With the advantage of neutron's high sensitivity to Ni and Mn, the two cations are definitely differentiated. The crystal structures as well as the occupancies of Mn and Ni are determined using Rietveld refinement, so that the ordering degree tuning up by annealing time is verified. However, the PDF reveals that the disordered model with Fd-3m space group does not fit the local structure. The M-O bonds (M = Mn or Ni) has dependence only on the type of cation but not on the ordering degree. In the view of the local structure of LiNi_0.5Mn_1.5O₄, the environment of LiO₄ tetrahedral and MO₆ octahedral are redrawn. The electrochemical properties and cycling behaviors of the LiNi_0.5Mn_1.5O₄ cathodes with various ordering degrees will be discussed and linked with the local structures.