Llz Oxide and LPS Sulfide Composite Solid Electrolyte for Lithium Ion Battery
Authors: Ohta Shingo , Yada Chihiro , Saito Toshiya , Iba Hideki 
Published: February 2020

Lithium ion secondary batteries have been widely used as power sources for electric vehicle (EV, PHV), a mobile phone and etc., and are now indispensable for our daily lives. As a result, a demand for large energy densities of the batteries is straightly increased. However, commercial lithium-ion batteries suffer some limitations considering their safety for using an organic liquid electrolyte because of its flammable.

All-solid-state batteries have been focused as a safer alternative candidate with a non-flammable solid electrolyte. Especially, solid sulfide electrolytes have been focused as an advanced electrolytes as the following reasons: (1) Generally, sulfides have low bond dissociation energy and low ion packing density, so all-solid-state batteries using sulfide electrolytes can be easily form by only uniaxial-press at room temperature. (2) Li10GeP2S12 (LGPS) and its family show remarkably high lithium ion conductivity (~ 10-2 Scm-1), which is comparable to that of an organic liquid electrolyte. However, some in the sulfide electrolytes is not stable in air and thus release toxic gases (e.g. H2S) in case of breaking down. Therefore, increasing attention has been paid to the use of oxide electrolytes due to their good chemical stability in air. However, lithium ion conductivity of oxide is lower than that of sulfide. Furthermore, oxides have high bond dissociation energy and all-solid-state batteries cannot be form by only uniaxial-press at room temperature. In other words, the solid electrolyte having (1)high ion conductivity, (2)high safety, (3)easily formability, does not appear yet.

In order to overcome this difficulty, our strategy is the design of the electrolyte having the requirements of each property (e.g. lithium ion conductivity, safety, and formability) by using oxides and sulfides composite (Figure). Oxides are “bulk grain” in the composite electrolyte to assure a safety of the battery. And sulfides are “grain boundary” in the composite electrolyte to keep the good formability of the battery. In order to develop the composite electrolyte for practical application (e.g. EV and PHV), It is strongly important to the design of the interface between oxides and sulfides because ion conduction is often inhibited in the hetero-interface. Thus, one of the focus points of this study is the improvement of the interface resistance. It is required for the development of the oxides and sulfide to achieve this requirement. Garnet-type oxides Li7La3Zr2O12 (LLZ) and its family have been expected to one of the promising candidate for oxide electrolytes because they have larger lithium ion conductivity (~10-3 Scm-1) in oxide materials. Furthermore, they show lower resistance between grain boundaries. So, it would be expected to form the lower resistance interface with sulfide. LGPS and LPS (XLi2S – (1-X)P2S5) series would be a candidate for sulfide electrolyte because of their high lithium ion conductivity and good formability.

As a result of this study, the interface resistance between oxides and sulfides depend on their composition and it is dynamically changed. This interface resistance would affect directly to the lithium ion conductivity for the composite electrolyte. The investigation of the interface performance leads to improve the interface resistance to neglected level, and then lithium ion conductivity of the composite electrolytes was successfully enhanced from ~10-6 Scm-1 to ~ 10-3 Scm-1.

Figure 1