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

Wednesday, May 14, 2014
Grand Foyer, Lobby Level (Hilton Orlando Bonnet Creek)
Y. Chen (Chemical and Engineering Materials Division, Oak Ridge National Laboratory), K. An (Oak Ridge National Laboratory), and C. Liang (Center for Nanophase Materials Sciences, Oak Ridge National Laboratory)
The spinel-type LiNi0.5Mn1.5O4 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 LiNi0.5Mn1.5O4 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 LiNi0.5Mn1.5O4, the environment of LiO4 tetrahedral and MO6 octahedral are redrawn. The electrochemical properties and cycling behaviors of the LiNi0.5Mn1.5O4 cathodes with various ordering degrees will be discussed and linked with the local structures.