The crystal structure of LLZ
The crystal structure of LLZ

Japan Fine Ceramics Center (JFCC) found that it is possible to synthesize a solid electrolyte at low costs.

Low-cost solid electrolytes are crucial in commercializing all-solid-state lithium-ion (Li-ion) rechargeable batteries, which are expected to be commercialized in the near future. This time, JFCC developed a technology to synthesize Li7La3Zr2O12 (lithium·lanthanum·zirconium·oxygen, LLZ), which is an oxide ion conductive material expected to be a major candidate for solid electrolyte, at low temperatures, creating a potential to lower costs.

LLZ, which is a Li-ion conductive oxide, is an oxide stable to water and Li (metal). Therefore, it is considered as a major candidate for the solid electrolyte material of all-solid-state Li-ion rechargeable battery. In general, LLZ is made by using a synthesis method using solid phase reaction and a mixture of Li2CO3 (carbonate), La2O3 (oxide) and ZrO2 (oxide) as a starting material.

After examining the details of the reaction with the synthesis method, the Material Processing Group of JFCC's Materials Research and Development Laboratory announced that it results in a single phase of LLZ through two kinds of synthesis reaction paths.

In one path, an intermediate product (La2Zr2O7) is generated at a temperature of 650°C, and it becomes LLZ at a temperature of 700°C. In the other path, an intermediate product (Li2ZrO3) is generated at a temperature of 700°C, and it becomes LLZ at a temperature of 900-1,250°C. Both of the paths result in a single phase of LLZ.

In consideration of the reaction paths, the Material Processing Group proposed a new synthesis method using La2Zr2O7 as a starting material. By synthesizing a mixture of La2Zr2O7 and Li2CO3 (Li material), which are available at relatively low costs, in a high-temperature solid-phase reaction (750°C, one hour), single-phase LLZ powder can be obtained, the group said.

"Unlike conventional synthesis methods, the new method does not require high temperatures (900-1,250°C) and enables to lower costs," it said.

The single phase of LLZ was confirmed by powder X-ray diffractometry. The method of forming a structure that has a framework made of oxide compound and inserting Li atoms from Li material hardly causes the volatilization of Li because of the low temperature of 750°C, eliminating the need to use too much Li, the group said.

It has already applied for a patent for the new synthesis method in Japan.