New High-Capacity Positive Electrode Materials on Li-Ni(II)-Nb(V) System

Abstract

High-capacity positive electrode materials are needed to further increase energy density of Li-ion battery. Numerous materials have been studied as positive electrodes. Among them, Li2MnO3-based electrode materials are generally recognized as the most promising high-capacity positive electrode materials for rechargeable lithium batteries. Recently, our group has reported that Li3NbO4,[1] which has higher lithium contents than that of Li2MnO3, is potentially utilized as host structures for a new series of high-capacity electrode material. Although Li3NbO4 crystallizes as the lithium-excess rocksalt-type structure, Li3NbO4 is electrochemically inactive because of the absence of electrons in a conduction band (4d0 configuration for Nb5+). Transition metals substituted for Li+ and Nb5+ donate electrons in the conduction band. Mn3+-substituted Li3NbO4 delivers large reversible capacity (approximately 300 mAh g-1) with highly reversible solid-state redox reaction of oxide ions.[1] In this study, crystal structures and electrode performance of a binary system between Li3NbO4 and NiO (x Li3NbO4 – (1 – x) NiO) are systematically examined as potential new high-capacity electrode materials. Binary system of xLi3NbO4 - (1-x) NiO was prepared from Li2CO3 (98.5 %, Kanto Chemical CO., Inc.), Nb2O5 (99.9 %, Wako Pure Chemical Industries, Ltd.), and NiCO3 • 2Ni(OH)2 • 4H2O (Wako Pure Chemical Industries, Ltd.). Composite electrodes consisted of 80 wt% active materials, 10 wt% acetylene black, and 10 wt% poly(vinylidene fluoride), pasted on aluminum foil as a current collector. Metallic lithium was used as a negative electrode. The electrolyte solution used was 1.0 mol·dm−3 LiPF6 dissolved in ethylene carbonate and dimethyl carbonate (3:7) (Kishida Chemical CO., Ltd). A polyolefin membrane was used as a separator. Electrode performance of samples was evaluated in Li cells at a rate of 5 mA g- 1 at room temperature or 50 oC. Figure 1a shows X-ray diffraction patterns of different samples found in x Li3NbO4 – (1 – x) NiO binary system. All samples assigned to rocksalt-related structures. Samples of x = 0.33 and 0.36 are found to be isostructural with Li3Ni2NbO6, which has cation ordering of Nb and Li/Ni.[3] In addition, the sample of x = 0.44 is found to be cation-disordered rocksalt-type structure. Electrochemical properties of the samples in Li cells are compared in Figure 1b. Although the sample (x = 0.33) shows small capacity (100 mAh g-1), high discharge voltage based on Ni2+/Ni4+ is observed. In contrast, the sample (x = 0.44) shows high reversible capacity (200 mAh g-1), but huge polarization on charge/discharge is observed. Together with these results, we will further discuss the possibility of x Li3NbO4 – (1 – x) NiO binary system as positive electrode materials with high energy density for rechargeable lithium batteries.