Li₂MnO₃-based electrode materials have been extensively studied as positive electrode materials in the past decade. The reaction mechanism of this material had been the controversial subject for a long time. Since the oxidation state of manganese ions is tetravalent, oxidation of manganese ions beyond the tetravalent state is difficult in Li cells. Instead of manganese ions, negatively charged anions, oxide ions (O^2-), donate electrons on charge. However, oxidation of oxide ions results in partial loss of oxygen as an irreversible process, i.e., decomposition reaction.

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[1] and Li₄Mo⁶⁺O₅[2], 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, Li_1.3Nb_0.3Mn_0.4O₂ (0.43Li₃NbO₄ – 0.57LiMnO₂), delivers large reversible capacity (approximately 300 mAh g^-1) with highly reversible solid-state redox reaction of oxide ions.[1]

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 (Li_1.2Ti_0.4Mn_0.4O₂) was prepared from Li₂CO₃, TiO₂ (anatase-type), and Mn₂O₃. As-prepared Li_1.2Ti_0.4Mn_0.4O₂ 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 Li_1.2Ti_0.4Mn_0.4O₂ sample is shown in Figure 1a, and all diffraction lines are assigned to cation disordered rocksalt-type structure. Ball-milled Li_1.2Ti_0.4Mn_0.4O₂ 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 Li_1.2-xTi_0.4Mn_0.4O₂ quite resembles that of Li_1.3-xNb_0.3Mn_0.4O₂. Available energy density of Li_1.2-xTi_0.4Mn_0.4O₂ 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.

Acknowledgements

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.”

References

[1] N Yabuuchi et al., Proceedings of the National Academy of Sciences, 112, 7650 (2015).

[2] N. Yabuuchi et al., Chemistry of Materials, in-press, DOI: 10.1021/acs.chemmater.5b04092.