Unveiling the Influence of Percolation Network on Accurate Measurement of Electrochemical Stability in Solid Electrolytes

Abstract

All-solid-state batteries (ASSBs) are garnering attention as the next-generation battery. The solid electrolytes (SEs), a crucial component of ASSBs, necessitates a wide electrochemical stability window (ESW) to achieve long-term stable solid-state batteries with high energy density. In this respect, ESW is an important indicator that enables to priorly identify whether the solid electrolyte decomposes in battery system depending on voltage ranges or junction with cathode materials in thermodynamic perspective. Thus, precise evaluation on ESW is highly required as prerequisite to understand interfacial phenomena in solid-state batteries. Recently, carbon additives (C) have been commonly incorporated into the composite of working electrodes (SE/C) to measure ESW precisely by mitigating electronic kinetic limitation in linear sweep or cyclic voltammetry.[1] Systematic investigation on both electronic and ionic conduction in composite electrode for ESW evaluation is necessary given the mixed conducting property of composite, however, ionic conduction in composite is currently overlooked without thorough consideration. In this study, we reveal that it is crucial to balance the ionic and electronic conductivity of composite electrode for precise measurement of the intrinsic ESW by adjusting the ratio of carbon and SEs. Not only in terms of electronic conduction, ionic kinetic overpotential (limitation) drives significant differences in the onset voltage and electrochemical behavior depending on the ratio of carbon to SEs. These findings are particularly significant for halide-based SEs which are recently received great attention due to high oxidation stability (> 4V), because their anodic stability is close to the charging cut-off voltage of the cathode, indicating that even minor differences in the oxidation stability of the electrolyte can have a significant impact on capacity and cycle stability. Li3InCl6 (LIC), halide SE with an ionic conductivity of 1 mS/cm, shows significant differences in the onset voltage and the shape of the voltage profile depending on the ratio of carbon to SEs. The ratio LIC and C induces the difference in the oxidation/reduction stability, emphasizing the importance of the measuring condition of ESW to analyze the precise electrochemical characteristics of the halide SEs. The solid electrolyte decomposition behavior according to effective ionic/electronic conduction in LIC/C composite is investigated with galvanostatic current charging and cyclic voltammetry method, X-ray analysis (XRD & XPS), and density functional theory (DFT) calculations. Through a fundamental understanding of measuring the intrinsic ESW of SEs with minimized kinetic overpotentials, we aim to provide guidelines for designing more stable SEs and anticipate that this will lead to a clearer understanding of degradation mechanisms influencing cycle stability.