Li2s-P2S5 Solutions for Forming Solid Electrolyte Coating Layers on Electrode Materials for All-Solid-State Batteries

Abstract

Sulfide-based solid electrolytes are promising for all-solid-state batteries because they exhibit high ionic conductivity over 10−4 S cm−1 and have good ductility. They are often prepared by mechanical milling or high-temperature synthesis. As an alternative to these methods, the preparation of sulfide solid electrolytes by a liquid-phase process, using organic solvents to promote the reactions, has been proposed during the last few years. The liquid-phase synthesis is a more facile process for industrial scaling-up. It also offers the possibility to produce solid electrolyte coating layers on electrode materials by their direct precipitation, achieving intimate contacts, which is crucial for good electrochemical performance in all-solid-state batteries.Although great attention has been given to the liquid-phase synthesis of sulfide solid electrolytes during the last years, the reaction mechanisms and the role of solvents in the reactions is not yet well understood. In this work, we elucidated the reaction between Li2S and P2S5, mediated by acetonitrile that leads to the formation of the PxSy z- thiophosphate anions.Li2S and P2S5 in a 1:1 molar ratio were found to quickly react to form a LiPS3 solution. The 31P NMR spectrum of the LiPS3 solution (Figure 1) showed that the P atom was located in different chemical environments, indicating the formation of polymer-like [PS3 −]n chain units. The [PS3 −]n chains react upon heat treatment at temperatures above 180 °C to form P2S6 2− units, as it was confirmed by Raman and 31P MAS NMR spectroscopy. However, an increase in the sulfur content by adding Li2S to the LiPS3 solution was found to break the P-S-P bonds in the [PS3 −]n chains, resulting in the formation of PS4 3− units. If sulfur provided by Li2S is not enough to break all P−S−P bridges, the PS4 3− units and the remaining [PS3 −]n chains react upon heat treatment (>180 °C), resulting in the formation of P2S7 4− units [2]. These findings give a deeper insight into the reaction mechanisms governing the liquid-phase synthesis of sulfide solid electrolytes. Moreover, the characterization of the LiPS3 solution indicates it as a promising material for the formation of solid electrolyte coating layers on electrode materials.