(Invited) Influences of Interfaces on Performance of Solid-State Batteries
Authors: Takada Kazunori 
Published: February 2020

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 for large batteries; however, the current liquid electrolytes are not stable enough to meet this 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. On the other hand, the highest ionic conductivities have reached 10−2 S cm−1 among sulfides, 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 a solid-state battery than in liquid systems [1].
The high-power solid-state battery indeed employ a solid electrolyte with high ionic conductivity of 2.5 × 10−2 S cm−1; however, it also uses some materials that are not highly-conductive curiously [2]. Surface of the cathode material, LiCoO2, is covered with LiNbO3, although the ionic conductivity of LiNbO3 is only of the order of 10−6 S cm−1. In addition, Li4Ti5O12 is used as the anode in the batteries, although it is poor in ionic conduction and electronically insulating. The reasons for their use can be understood by anomalous transport properties at the interfaces. Although some sulfide electrolytes exhibit high ionic conductivity, they show so high resistance at the interface to high-voltage cathodes that LiNbO3 interposed at the interface lowers the interfacial resistance in spite of its low ionic conductivity. The LiNbO3 eliminates the origin of the huge resistance, which is lithium depletion at the interface, because it is an oxide-based solid electrolyte. On the other hand, electrode reactions promote mass transfer. The lithium insertion does not generate interstitials that contribute to ionic conduction, because it proceeds under a two-phase reaction between Li4Ti5O12 and Li7Ti5O12. Moreover, the Li7Ti5O12 is also a bad conductor; however, the electrode reaction is promoted due to the enhanced transport at the Li4Ti5O12/Li7Ti5O12 interface [3].
[1] Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba. R. Kanno, Nat. Energy 1, 16030 (2016).
[2] K. Takada, T. Ohno, N. Ohta, T. Ohnishi, and Y. Tanaka, ACS Energy Lett., 3, 98 (2018).
[3] Y. Tanaka, M. Ikeda, M. Sumita, T. Ohno, and K. Takada, Phys. Chem. Chem. Phys., 18, 23383 (2016).