Ionic Mott-Schottky formalism allows the assessment of mobile ion concentrations in Li+-conducting solid electrolytes

Abstract

• Classical Mott-Schottky theory can be transferred from semiconductors to solid electrolytes. • With few adaptions, it can describe the ionic space-charge capacitances in blocking electrodes. • From impedance spectra and a physical equivalent circuit, the mobile charge carrier concentration can be calculated. • The analysis reveal that only a fraction of the stoichiometric Li-ions take part in the conduction process. Li + -conducting solid electrolytes have been in the focus of researchers for the last decade thanks to the recent efforts to develop efficient all-solid-state batteries. While material scientists discovered new compounds with unprecedented Li + conductivities, surprisingly often, a fundamental question remains: What fraction of the total lithium-ions inside the solid electrolyte is mobile and thus contributes to the conductivity? In this work, we propose an approach, which can be considered as a Mott-Schottky-based analysis applied to an ionically conducting system, to calculate the concentration of mobile Li + in a lithium-conducting glass–ceramic. We use the space charge layer (SCL) capacitances obtained from the electrochemical impedance data recorded at different bias potentials and temperatures as the input. We demonstrate that the space charge layer in the ionic conductors can be formally described similarly to the SCLs formed in semiconductors in contact with, e.g., liquid electrolytes. This analysis makes it possible to provide deeper insights into the mechanisms of conductivity and its temperature dependencies in the SCL in ion conductors and, in general, help to uncover the nature of the mobility of Li-ions in various Li + -conducting electrolytes.