Solution-Processable Li and Na Superionic Conductors for All-Solid-State Batteries

Abstract

Bulk-type all-solid-state lithium batteries using sulfide solid electrolytes are considered a very promising solution for tackling the safety challenges associated with conventional lithium-ion batteries. However, further development of solid electrolytes is imperative in order to improve their ionic contacts with active materials, conductivity, scalability of synthesis protocols, and air-stability. A solution-based synthesis process can provide a breakthrough in the architecture and fabrication of composite structures. This study show that a new, highly conductive (4.1×10-4 S cm-1 at 30oC), highly ductile, and dry-air-stable glass 0.4LiI-0.6Li4SnS4 is prepared at 200oC using a scalable method that employs a homogeneous methanol solution. Comprehensive diagnostic analyses reveal that lowering the crystallinity and incorporating large and highly polarizable iodide ions into Li4SnS4 improve the ductility and conductivity. Importantly, the solution process enables the wetting of any exposed surface of the active materials with highly conductive solidified electrolytes (0.4LiI-0.6Li4SnS4), resulting in considerable improvements in electrochemical performances of these electrodes over conventional mixture electrodes. Even though sodium-ion batteries (NIBs), which is another important class of battery type, have been developed extensively due to the advantage of low cost, development of all-solid-state Na batteries (ASNBs) has remained challenging because of relatively low ionic conductivity of Na-ion conductor. Na3SbS4 show high conductivity of 1.1×10-3 S cm-1 which is one of the most promising result so far. Furthermore it remain its structure after dissolving to water or methanol with moderate ionic conductivities. Consequently, sodium-ion conductive coating layers were casted to active material successfully. The results hold great promise for practical all-solid-state technology as well as provide insights into discovering broad classes of solution-processable superionic conductors.