Ionic conductivity in glasses: fundamentals and application to solid state batteries

Abstract

Understanding ionic transport in solids is critical for advancing solid-state electrolytes, where Ionic conductivity is mainly the product of the effective charge carrier density and its mobility. However, due to the low mobility of ions in solid-state, measuring the individual contributions of mobility and effective charge carrier density represents an experimental challenge. In this work, electrochemical impedance spectroscopy combined with space charge polarization theory is employed to independently quantify the effective charge carrier density in ion-conducting glasses. A systematic investigation of lithium disilicate glass establishes optimized experimental conditions and reveals that only a small fraction of alkali ions contributes to ionic conduction at room temperature. Extension of this methodology to lithium silicate and phosphate glasses demonstrates a composition-dependent two-regime behavior: at moderate alkali concentrations, conductivity enhancements are primarily governed by increases in effective charge carrier density, whereas near vitrification limits ionic mobility becomes the dominant factor. Additionally, the incorporation of chloride into Li2O–P2O5–AlCl3 glasses significantly enhances ionic conductivity. Since the addition of AlCl3 does not introduce new cationic charge carriers such as Li+, the observed enhancement in ionic conductivity must instead originate from an interaction between chloride anions and the glass network. Finally, these fundamental insights are translated into application through the development of a solvent-free, photo-cross-linked glass–polymer composite electrolyte, which exhibits improved electrochemical stability, reduced interfacial resistance, and superior performance in all-solid-state lithium battery cells compared to conventional polymer electrolytes.