Understanding the impedance spectra of all-solid-state lithium battery cells with sulfide superionic conductors

Electrochemical impedance spectroscopy (EIS) will assist the development of all-solid-state lithium batteries by identifying their performance limiting resistances, although an elaborate distinction method has not been established to date. Herein, the distribution-of-relaxation-times method was used to support the quantification and understanding of EIS data, which were compiled over various temperatures and states of charge (SOCs) for all-solid-state cells with an In–Li anode, coated-LiCoO2 composite cathode, and separator comprising two kinds of sulfide solid electrolytes (SEs): Li10+xGe1+xP2−xS12 (LGPS) or its structural analogue in the Li–P–S–Br system (LPSBr). It was revealed that the In–Li interface and interphase resistances differ significantly depending on the separator SE, confirming an imperceptible chemical reaction between In–Li/LPSBr and the chemical instability of In–Li/LGPS. Consequently, the LGPS-based cell suffers from a large total impedance (188 Ω at 298 K and 100% SOC with 151 Ω at Li–In/LGPS), which is 250% higher than that of the LPSBr-based cell (66 Ω with 18 Ω at Li–In/LPSBr). This study provides guidance for the quantitative analysis of EIS data and in improvement of battery performance, such as developing new SEs that are as ion-conductive as LGPS and as stable as LPSBr.