Enhancement of the High‐Rate Capability of Solid‐State Lithium Batteries by Nanoscale Interfacial Modification

Abstract

Rechargeable lithium batteries are widely used in portable equipment today. However, there have always been safety issues arising from their combustible organic electrolytes. These issues are becoming more serious with the increasing size of batteries for use in electric vehicle (EV) or load-leveling applications. Nonflammable solid electrolytes would be the ultimate solution to the safety issue. Despite their high safety, the energy densities and power densities of solid systems have been too low for their practical use. We have succeeded in increasing their energy densities to levels comparable to those of liquid ones. However, power densities, or high-rate capabilities, remain poor. In this communication, we report that a buffer film with a thickness of only several nanometers interposed between the electrode and electrolyte materials improves the high-rate capability of solid-state lithium batteries. Low ionic conductivities of solid electrolytes have been the reason for the poor high-rate capability of solid-state lithium batteries. Although the conductivities of recently discovered solid electrolytes (> 10 S cm) are slightly lower than those observed for liquid electrolytes, Li ionic conduction in the solid electrolytes has become as fast as that of liquid electrolytes, by taking into account the fact that the transport number of ions in inorganic electrolytes is unity. However, the high-rate capability of solid systems, including solid electrolytes, remains inferior. This fact strongly suggests that the rate-controlling step is not in the bulk of the solid electrolytes, but at the interface between the electrode and the electrolyte materials. Ionic conduction at interfaces between different kinds of ionic conductors, or heterojunctions, is characterized by the term “nanoionics”; a frontier study was done for a LiI– Al2O3 composite, [7] and a sophisticated example was presented by Sata et al. In the latter, two kinds of F ion conductors, BaF2 and CaF2, were brought into contact with each other. Part of the F ions then transferred from one side to the other to reach an equilibrium, which produced vacancies in the former and interstitial ions in the latter, both of which contributed to ionic conduction at the interface and enhanced the ionic conduction. Similar nanoionic phenomena should take place at the interface between the electrode and the electrolyte materials, forming a space-charge layer. Because the compositions and structures of solid electrolytes have been well tailored to achieve high ionic conductivities, the ionic conductivity of the space-charge layer, where the compositions deviate from the optima, should be lower than that of the bulk, increasing the interfacial resistance. For instance, the Li4GeS4–Li3PS4 (thio-LISICON) system used in the present study has a high ionic conductivity of the order of 10 S cm at its optimum composition. However, variation in the composition reduces it to 10 S cm. The detrimental increase in the interfacial resistance would be prominent in bulk-type or non-thin-film solid-state lithium batteries. Sulfide electrolytes should be used in such batteries, because the ionic conductivities of oxide solid electrolytes, for example, are so low that they are available only in thin-film systems. On the other hand, the cathode should be an oxide, such as LiCoO2, because of its high electrode potential. [10,11]