The use of anion redox, especially oxide ions, is a crucial strategy to design and develop new electrode materials with high gravimetric/volumetric energy density for rechargeable lithium batteries. Reversible capacity of electrode materials is potentially further increased by the enrichment of lithium contents with less transition metals in the close-packed structure of oxide ions. Recently, our group has reported that Li3Nb5+O4 and Li4Mo6+O5, which have higher lithium contents than those of Li2MnO3, are potentially utilized as host structures for a new series of high-capacity electrode materials. Among them, Mn3+-substituted Li3NbO4, Li1.3Nb0.3Mn0.4O2 (0.43Li3NbO4 – 0.57LiMnO2), delivers large reversible capacity (approximately 300 mAh g-1) with highly reversible solid-state redox reaction of oxide ions.
In this study, Li2Ti4+O3 is targeted and revisited as the host structure for high-capacity electrode materials. Mn3+-substituted sample, 0.5Li2TiO3 – 0.5LiMnO2 (Li1.2Ti0.4Mn0.4O2) was prepared from Li2CO3, TiO2 (anatase-type), and Mn2O3. As-prepared Li1.2Ti0.4Mn0.4O2 was mixed with 10 wt% acetylene black (HS-100, Denka Co. Ltd) and ball-milled to enhance the electrode performance. An X-ray diffraction pattern of the ball-milled Li1.2Ti0.4Mn0.4O2 sample is shown in Figure 1a, and all diffraction lines are assigned to cation disordered rocksalt-type structure. Ball-milled Li1.2Ti0.4Mn0.4O2 shows large reversible capacity as shown in Figure 1b, and the Nb-free sample delivers more than 300 mAh g-1 at 50 oC. A voltage profile of Li1.2-xTi0.4Mn0.4O2 quite resembles that of Li1.3-xNb0.3Mn0.4O2. Available energy density of Li1.2-xTi0.4Mn0.4O2 exceeds 1,000 mWh g-1as a positive electrode material, which shows acceptable capacity retention as shown in Figure 1b. Moreover, charge compensation is realized by oxidation of oxide ions, and formation of peroxide-like species is evidenced by O K-edge X-ray absorption spectroscopy.
From these results, we will discuss the possibility of high-capacity positive electrode materials, which effectively use the solid-state redox of oxide ions for the charge compensation, consisting of only 3d-transtion metals.
This research has been partly supported by Advanced Low Carbon Technology Research and Development Program of the Japan Science and Technology Agency (JST) Special Priority Research Area “Next-Generation Rechargeable Battery.”
 N Yabuuchi et al., Proceedings of the National Academy of Sciences, 112, 7650 (2015).