Lithium-ion batteries have been used in portable equipment as a key component in an information society. In addition, much larger batteries are now required for energy storage in vehicles and smart grids. We have to tackle the inherent issues, which originate from non-aqueous electrolytes, in lithium-ion batteries again for developing the large batteries. The increasing size makes the safety issues more serious due to the increasing amounts of flammable electrolytes and the lowering heat radiation. Moreover, much longer durability is required; however, the current liquid electrolytes are not stable enough to meet the requirement. Solid electrolytes are expected to provide fundamental solutions to these issues.
Solid-state batteries had been suffering from the low power density due to low ionic conductivities of solid electrolytes. However, the highest ionic conductivities have reached 10−2 S cm−1 among sulfide, which is comparable to, or even higher than that of the liquid electrolytes, when the transport number of unity is taken into account. Since ionic conductivities of solid electrolytes have become high enough, interfaces between the battery materials are now playing critical roles in battery performance, which can be recognized in a recent paper that reports higher power density in solid-state batteries than in liquid systems .
In the solid-state batteries reported therein, surface of the cathode material, LiCoO2, is covered with LiNbO3, although the LiNbO3 is not highly-conductive. Because sulfide electrolytes exhibit very high resistance at the interface to high-voltage cathodes, even such a poor ionic conductor lowers the interfacial resistance by eliminating the origin of the huge resistance, when it is an oxide-based solid electrolyte. In addition, Li4Ti5O12 is used as the anode in spite of the low transport properties. The reason can be understood from computation results suggesting that electronic and ionic transports are enhanced at the Li4Ti5O12/Li7Ti5O12 interface.
 Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba. R. Kanno, Nat. Energy 1, 16030 (2016).