Domain Modeling of Lithium-Rich Manganese Nickel Oxide By Reverse Monte Carlo Method

INTRODUCTION

   Lithium manganese nickel oxides with layered structures have been investigated actively in the last decade. Especially, materials with lithium-rich compositions, such as Li_1.2Mn_0.6Ni_0.2O₂, are regarded as promising candidates for a cathode active material of the lithium-ion battery, due to higher energy densities, lower costs than typical commercialized cathodes like LiCoO₂-based materials. It is well-known that their electrochemical properties are affected by cation distributions in the crystals, such as a LiMn₆domain in a transition-metal layer and a Ni-O domain due to a cation mixing of Li and Ni. Therefore, the domain structures have been studied recently by TEM, NMR, extended X-ray absorption fine structure (EXAFS) and so on.

   For deeper understanding of domain structures, we have focused on the Reverse Monte Carlo (RMC) modeling on crystals using the Bragg reflections and total scatterings simultaneously. In this work, we applied the RMC to lithium manganese nickel oxides with layered structures and then determined local (non-periodic) domain structures in intermediate range up to ca.10 Å in addition to average (periodic) host structures.

EXPERIMENTAL

   Lithium transition-metal oxides, such as Li_1.2Mn_0.6Ni_0.2O₂, were synthesized by a coprecipitation method. Phase identifications of the samples were carried out laboratorial X-ray diffraction measurements, and the metal compositions were investigated by the inductively-coupled plasma (ICP) technique. For the reverse Monte Carlo modeling on crystals, synchrotron X-ray diffraction/total scattering patterns were measured by BL02B2 and BL04B2 installed at SPring-8, Japan. By using both the Bragg profile and structure factors S_box(Q) which were degraded taking simulation-box sizes into account, we performed the RMC simulation with RMCProfile, and then analyzed domain structures of the specimens.

RESULTS AND DISCUSSION

   X-ray diffraction and ICP measurements confirmed that the Li_1.2Mn_0.6Ni_0.2O₂ had a single phase of the Li₂MnO₃-type layered structure and the analytical metal composition was almost equal to the nominal value.

   For the purpose of a domain modeling in the crystal, we measured both the Bragg profile with high reciprocal-space resolution and the total scattering pattern with wide Q range. By using both the Bragg data and the convolved S_box(Q), the RMC simulation was carried out. Figure 1 shows results of the RMC for Li_1.2Mn_0.6Ni_0.2O_2. It is demonstrated that the RMC modeling can be performed successfully. From the simulated atomic configuration, we obtained pair distribution functions g(r) and then analyzed them. As a result, it is found that there is a correlation around 4 Å in g_Ni-Ni(r). This indicates that a Ni-O domain is formed in the crystal.