Dependence on lithium-ion batteries for automobile applications is rapidly increasing. The emerging use of anionic redox can boost the energy density of batteries, but the fundamental origin of anionic redox is still under debate. Moreover, to realize anionic redox, many reported electrode materials rely on manganese ions through π-type interactions with oxygen. Here, through a systematic experimental and theoretical study on a binary system of Li 3 NbO 4 −NiO, we demonstrate for the first time the unexpectedly large contribution of oxygen to charge compensation for electrochemical oxidation in Ni-based materials. In general, for Ni-based materials, e.g., LiNiO 2 , charge compensation is achieved mainly by Ni oxidation, with a lower contribution from oxygen. In contrast, for Li 3 NbO 4 −NiO, oxygen-based charge compensation is triggered by structural disordering and σ-type interactions with nickel ions, which are associated with a unique environment for oxygen, i.e., a linear Ni−O−Ni configuration in the disordered system. Reversible anionic redox with a small hysteretic behavior was achieved for LiNi 2/3 Nb 1/3 O 2 with a cation-disordered Li/Ni arrangement. Further Li enrichment in the structure destabilizes anionic redox and leads to irreversible oxygen loss due to the disappearance of the linear Ni−O−Ni configuration and the formation of unstable Ni ions with high oxidation states. On the basis of these results, we discuss the possibility of using σ-type interactions for anionic redox to design advanced electrode materials for highenergy lithium-ion batteries.

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. References [1] N Yabuuchi et al., Proceedings of the National Academy of Sciences, 112, 7650 (2015). [2] G. C. Mather and A. R. West, Journal of Solid State Chemistry, 124, 214, (1996). Figure 1. (a) X-ray diffraction patterns and (b) charge/discharge curves of x Li3NbO4 – (1 – x) NiO binary system, (A) x = 0.33, (B) x = 0.36, (C) x = 0.44. Figure 1