Aqueous synthesis of complex hydride solid electrolytes for all-solid-state batteries

Abstract

The development of solid electrolytes for lithium batteries is pivotal in advancing battery technology, particularly for applications requiring high energy density and safety. Closo-type complex hydrides, known for their high ionic conductivity and structural stability, present a promising class of materials. Herein, we demonstrate the aqueous synthesis of superionic-conducting complex hydride solid electrolytes based on the ionic bonding nature between lithium cations and complex anions. Lithium complex hydrides containing closotype complex anions dissociate into lithium cations and complex anions in an aqueous solvent without decomposition and recrystallize to the pristine structure after the removal of the water solvent. These characteristics, in the context of solid electrolytes, allow the formation of structures identical to those produced via solid-state synthesis at lower synthesis temperatures (200 °C), achieving high ionic conductivity (6.6 × 10−3 S cm−1 ). Moreover, the high ionic conductivity, low material density, and soft nature of complex hydrides facilitate efficient ionic and electronic transport within high S content (50 wt%) cathode composites for all-solid-state Li-S batteries. This is achieved through uniform dispersion and intimate contact within the composites, which is crucial for reliable battery performance. Therefore, we utilized the water stability of complex hydrides for the aqueous synthesis of cathode composites, highlighting the importance of understanding the interactions between cathode composite materials and water. Our study offers insights into the design of water-stable complex hydride materials and the importance of designing cathode composites considering the three phases of sulfur-complex hydride-carbonaceous materials (S-CH-C).