Development of high energy batteries has been aspired all over the world to expand the usage in vehicles. Performance of Li-ion batteries (LIB) has been continuously improved since launched on the market. Quite recently a high energy battery pack over 300 Wh/kg was reported. A history from Li secondary battery to LIB tells us the merit of electrode materials with intercalation/insertion mechanism for sufficient life of the battery.

We may have to discuss about how much energy is actually required, but higher the energy density, more variety of the balance between each performance could be realized. Design of the systems would be easier if we could withdraw the theoretical restriction of LIB. Now conversion type electrodes attract much attention.

　Author’s group has developed the technique to increase energy of positive electrode for LIB, by surface modification and analogue of Li-excess layered materials. The latter seems closely related to the conversion mechanism. Then we focus on metal polysulfides such as TiS₄ and Li₅FeS₈ with specific capacity of more than 700 mAh/g by an intermediate mechanism between insertion and conversion [1, 2]. FeF₃ related positive electrodes show maximum energy density as closed system, and are operated by full conversion reaction to 3LiF + Fe for this purpose. This is more challenging and several technical barriers should be cleared. We hope to understand how to balance the energy density and other important properties through the materials we developed.

References

[1] A. Sakuda, H. Sakaebe, T. Takeuchi, et al., J. Am. Chem. Soc., 139, 8796 (2017) .

[2] T. Takeuchi, H, Sakaebe, et al., Solid State Ionics, 320, 378 (2018).

Acknowledgement

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under the “Research and Development Initiative for Scientific Innovation of New Generation Batteries 2 (RISING2)”