Study on Local Chemical Orderings in Li3NbO4-Based Positive Electrode Materials with a Disordered Rock-Salt Structure By a Reverse Monte Carlo Modeling

Abstract

1. Introduction As a positive electrode material for a lithium-ion battery, lithium-rich transition-metal oxides have drawn much attention due to the high discharge capacity. Among the oxides, we have focused on Li1+xM1-xO2 with a disordered rock-salt structure such as Li3NbO4-based solid solution [1]. It is well known that all the cations occupy a same crystallographic site in the crystals. Due to different ionic radii and electronic states of the cations, however, it can be predicted that local structures (non-periodic structures) around different cation species must be distinguishable, and such hidden strucures must affect Li+ behavior in charge and discharge processes. Therefore, some previous works tried to uncover the local structures [2], but the structures are still under debete. This is because the structures without periodicity do not show prominent Bragg peaks in a diffraction profile.From such background, this work investigated local structures of the Li3NbO4-based materials, which can be also denoted as Li1+x(Nb, M)1-xO2, with the disordered rocksalt structure by a reverse Monte Carlo (RMC) modeling using total scattering data. Especially, we paid special attention on Li1.3Nb0.3Fe0.4O2 and Li1.3Nb0.43Ni0.27O2 as typical materials. Based on the modeling results, we discuss effects of the cation species on the local structures.2. Experimental The Li3NbO4-based materials were synthesized by a conventional solid-state reaction method, according to literature [1]. Metal compositions of the samples were analyzed by an inductively coupled plasma atomic emission spectroscopy, and valences of cations were estimated by X-ray absorption near edge structures (BL14B2, SPring-8). Synchrotron X-ray Bragg profiles were collected with BL19B2 (SPring-8), and the crystal structures (average structures) were refined by a Rietveld method. We also measured neutron and synchrotron X-ray total scattering data at NOVA (J-PARC) and BL04B2 (SPring-8), respectively. By using structure factors S(Q) and reduced pair distribution functions G(r) in addition to the Bragg profiles, we performed the RMC modeling with an RMCProfile software under a bond-valence-sum (BVS) constraint. In the modeling, we optimized cation distributions by swapping the cations.3. Results We made a supercell with 1000 atoms from a refined unit cell, and then performed RMC modeling on Li1.3Nb0.3Fe0.4O2 with the disordered rock-salt structure. As a result, we could obtain an atomic configuration which could reproduce X-ray Bragg profile, neutron G(r), neutron and X-ray S(Q). From the atomic configuration, it was found that a local distortion of NbO6 was larger than those of the others (LiO6 and FeO6) in the material. The large distortion should be induced by a smaller ionic radius of Nb5+ and/or a mismatch of electronic state of Nb5+. Furthermore, it was clarified from a coordination-number analysis that Li tended to be surrounded by Nb considerably in the disordered rock-salt structure. Such a tendency was also observed in an atomic configuration of Li1.3Nb0.43Ni0.27O2. From the results, it can be concluded that Li diffusion is disturbed by a cation with a higher valence in the disordered structure.