Researchers at the Korea Institute of Science and Technology (KIST) have developed a superionic halogen-rich Li-argyrodite (HRLA) that works well as a promising solid electrolyte (Li_5.5PS_4.5Cl_1.5) in all-solid-state batteries without a decreased intrinsic electrochemical window in various electrode configurations.

This HRLA exhibits the highest Li-ion conductivity among Li-argyrodites (10.2 mS cm^–1 at 25 °C, cold-pressed powder compact) reported so far and that delivers cell performance of 114.2 mAh g⁻¹ at 0.5 C. A paper on their work is published in the ACS journal Nano Letters.

Electrochemical performance of an ASSB cell with the Li_5.5PS_4.5Cl_1.5 solid electrolyte at 25 °C. Rate capability performance at various C-rates. One C-rate corresponds to a current density of 3.1 mA cm⁻¹ (i.e., 180 mA g−1). Credit: ACS, Jung et al.

All-solid-state battery technology has attracted great attention as a possible safer—and eventually, higher energy density—alternative to Li-ion batteries with liquid electrolytes. However, unlike a liquid electrolyte in which Li ions can move freely, solid electrolytes have a low Li-ion conductivity of 1/10 to 1/100 of that of a liquid electrolyte because the movement of Li-ions is confined within a rigid solid lattice. Resolving this is one of the most important and difficult challenges in the development of all-solid-state battery technology.

Dr. Hyoungchul Kim’s research team at KIST developed their solid electrolyte using a sulfide-based crystalline structure called argyrodite. Argyrodite is a crystal structure that was discovered in the mineral of Ag₈GeS₆ by Clemens Winkler in 1886. Recently, much research has been conducted on replacing cation sites with alkali metals such as Li and applying them to electrochemical devices.

Expectations were high for argyrodite utilization due to high Li-ion concentration and structural stability, but Li-ion conductivity remained below 4 mS/cm due to the structural uniqueness of Li-ions trapped in the octahedral cage in the argyrodite crystal.

The KIST research team developed a novel Li-ion pathway that crosses the octahedral cage by applying a technique for selectively substituting chlorine, a halogen element, at specific atomic positions.

In addition, the new synthesis method using ultimate-energy mechanical alloying (UMA) and rapid thermal annealing (RTA) using an infrared lamp reduces the processing time for a solid electrolyte material from several days to less than 10 hours.

In this study, there is great significance in developing a high-performance solid electrolyte material with excellent mass productivity. Synthesis of superionic sulfide materials through rapid process is very likely to be commercialized, and can be widely used in electric vehicles and energy storage system as a solid electrolyte in the future.

—Dr. Kim

The research was supported by the Dual Use Technology Program of the Institute of Civil Military Technology Cooperation granted financially resources from the Ministry of Trade, Industry and Energy and the Defense Acquisition Program Administration. This work was also supported by the Institutional Research Program of the Korea Institute of Science and Technology and the Technology Development Program to Solve Climate Changes of the National Research Foundation funded by the Ministry of Science and ICT.

Resources

• Wo Dum Jung, Ji-Su Kim, Sungjun Choi, Seongmin Kim, Minjae Jeon, Hun-Gi Jung, Kyung Yoon Chung, Jong-Ho Lee, Byung-Kook Kim, Jong-Heun Lee, and Hyoungchul Kim (2020) “Superionic Halogen-Rich Li-Argyrodites Using In Situ Nanocrystal Nucleation and Rapid Crystal Growth” Nano Letters 20 (4), 2303-2309 doi: 10.1021/acs.nanolett.9b04597