Development of All-Solid-State Lithium Secondary Batteries Using Garnet Solid Electrolyte Layer

Abstract

Lithium ion secondary batteries have been used as power sources of various devices. The electrolyte of the lithium ion battery is needed to have high lithium ionic conductivity and wide electric potential window. In addition, it is favored to be nonflammable to enhance the safety of batteries. The oxide solid electrolyte is generally nonflammable, but the most oxide electrolyte has narrower potential window and lower ionic conductivity. In late years lithium garnet type oxide solid electrolyte (e.g. Li7La3Zr2O12[1]) is gaining attention due to have high Li+ ionic conductivity and electrochemical stability from 0 to 5V vs. Li/Li+. The development of the all-solid-state batteries using this garnet type electrolyte has been carried out, but there are various issues. The main issue of the all-solid-state batteries using oxide electrolyte is high internal resistance of electrodes, which originates from a poor contact not only between electrolyte particle and active material particle, but also among electrolyte particles. We investigated all-solid-state lithium batteries using lithium oxide solid electrolyte ceramics layer (Li1.5Al0.5Ge1.5(PO4)3 or Li7La3Zr2O12). The internal resistance of electrodes was reduced by interposing sulfide solid electrolyte (Li2S-P2S5) between the oxide electrolyte layer and the active material (e.g. Sulfur or LiCoO2) (Figure 1). The cathode structure enabled the batteries to work at room temperature [2-4]. In this study, we have developed the garnet type lithium solid electrolyte which has higher ionic conductivity above 1.0 x 10-3 Scm-1at 25°C. The performance of the all-solid-state batteries, using the garnet electrolyte and the sulfide electrolyte which is interposed between the oxide electrolyte layer and the active material, will be presented.