Origin of Stabilization and Destabilization in Solid-State Redox Reaction of Oxide Ions for Rechargeable Lithium Batteries
Authors: Yabuuchi Naoaki , Sato Kei , Kobayashi Yuki , Nakayama Masanobu , Hashimoto Yu , Mukai Takahiro , Shiiba Hiromasa , Yamanaka Keisuke , Mitsuhara Kei , Ohta Toshiaki 
Published: February 2020

Li2MnO3-based 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, further oxidation of manganese ions is difficult in Li cells. Instead of manganese ions, negatively charged anions, oxide ions (O2-), 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. 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, 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.[1]
Recently, Li2Ti4+O3 is also proposed as the host structure for high-capacity electrode materials with redox reaction of oxide ions.[3] Mn3+-substituted sample, 0.5Li2TiO3 – 0.5LiMnO2 (Li1.2Ti0.4Mn0.4O2), also delivers large reversible capacity as shown in Figure 1a. A voltage profile of Li1.2-x
Ti0.4Mn0.4O2 quite resembles that of Li1.3-x
Nb0.3Mn0.4O2. Available energy density of Li1.2-x
Ti0.4Mn0.4O2 exceeds 1,000 mWh g-1 as a positive electrode material. Moreover, charge compensation is realized by oxidation of oxide ions as evidenced by O K-edge X-ray absorption spectroscopy (Figure 1b) as a reversible process. In contrast to the Mn system, an iron counterpart, xLi2TiO3 – (1 – x) LiFeO2 binary system, shows large polarization on charge/discharge,[4] which is similar to that of Li3NbO4-LiFeO2 binary system.[1] For these Fe-containing materials, oxidation of oxide ions seems to trigger oxygen loss as an irreversible process.
From these results, we will discuss the origin of stabilization and destabilization in solid-state redox reaction of oxide ions, and 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, M. Takeuchi, M. Nakayama, H. Shiiba, M. Ogawa, K. Yamanaka, T. Ohta, D. Endo, T. Ozaki, T. Inamasu, K. Sato, and S. Komaba, Proceedings of the National Academy of Sciences, 112, 7650 (2015).
[2] N. Yabuuchi, Y. Tahara, S. Komaba, S. Kitada, and Y. Kajiya, Chemistry of Materials, 28, 416 (2016).
[3] N. Yabuuchi et al., submitted
[4] S. L. Glazier, J. Li, J. Zhou, T. Bond, and J. R. Dahn, Chemistry of Materials, 27, 7751 (2015).

Figure 1