Short- and Long-Range Order in the Positive Electrode Material, Li(NiMn)0.5O2: A Joint X-ray and Neutron Diffraction, Pair Distribution Function Analysis and NMR Study

Abstract

The local environments and short-range ordering of LiNi_0.5Mn_0.5O₂, a potential Li-ion battery positive electrode material, were investigated by using a combination of X-ray and neutron diffraction and isotopic substitution (NDIS) techniques, ⁶Li Magic Angle Spinning (MAS) NMR spectroscopy, and for the first time, X-ray and neutron Pair Distribution Function (PDF) analysis, associated with Reverse Monte Carlo (RMC) calculations. Three samples were studied: ⁶Li(NiMn)_0.5O₂, ⁷Li(NiMn)_0.5O₂, and ⁷Li(NiMn)_0.5O₂ enriched with ⁶²Ni (denoted as ⁷Li^ZERONi_0.5Mn_0.5O₂), so that the resulting scattering length of Ni atoms is null. LiNi_0.5Mn_0.5O₂ adopts the LiCoO₂ structure (space group R3̄m) and comprises separate lithium layers, transition metal layers (Ni, Mn), and oxygen layers. NMR experiments and Rietveld refinements show that there is approximately 10% of Ni/Li site exchange between the Li and transition metal layers. PDF analysis of the neutron data revealed considerable local distortions in the layers that were not captured in the Rietveld refinements performed using the Bragg diffraction data and the LiCoO₂ structure, resulting in different M−O bond lengths of 1.93 and 2.07 Å for Mn−O and Ni/Li−O, respectively. Large clusters of 2400−3456 atoms were built to investigate cation ordering. The RMC method was then used to improve the fit between the calculated model and experimental PDF data. Both NMR and RMC results were consistent with a nonrandom distribution of Ni, Mn, and Li cations in the transition metal layers; both the Ni and Li atoms are, on average, close to more Mn ions than predicted based on a random distribution of these ions in the transition metal layers. Constraints from both experimental methods showed the presence of short-range order in the transition metal layers comprising LiMn₆ and LiMn₅Ni clusters combined with Ni and Mn contacts resembling those found in the so-called “flower structure” or structures derived from ordered honeycomb arrays.