Multiple-reaction kinetics of composite electrodes for sulfide-based all-solid-state batteries: Impedance decoupling

Abstract

All-solid-state batteries (ASSBs) have garnered considerable attention as next-generation energy-storage systems owing to their safety. In ASSBs, composite electrodes comprising solid electrolytes, active materials, and conductive additives are employed to increase the electrochemically active surface and ensure the Li⁺/electron pathways. Composite electrodes consist of numerous solid–solid contact interfaces and exhibit interfacial side reactions that inhibit charge transfer. Thus, a comprehensive analysis of their electrochemical processes is necessary to develop high-performance ASSBs. Herein, we demonstrate the quantitative analyses of the multiple-reaction kinetics in a solid composite electrode for ASSBs with state-of-the-art Li₆PS₅Cl electrolytes and LiNi_0.8Co_0.1Mn_0.1O₂ active materials using modified transmission line models based on networks of distributed impedance elements. Firstly, model simulations are conducted with different boundary conditions and circuit elements to investigate the impedance responses of electrodes. The circuit elements represent the interfacial and charge/atom transport processes, namely, Li⁺/electron transport, charge transfer, double-layer charging, and diffusion in the active material. Secondly, for quantitative experimental analyses, two model cells (i.e., symmetric cells and full cells) are fabricated with various electrode formulations, and their impedance spectra are measured as a function of temperature. Kinetic parameters are determined using the appropriate equivalent circuit models to elucidate the effects of electrode formulation and operating conditions on electrochemical performance. Finally, the interplay between the kinetics of ionic transport and interfacial reaction is discussed based on measurements using uncoated active materials and halide electrolytes. Thus, this study affords insights into the electrochemical processes of ASSBs and guidelines for quantitative analyses via the impedance decoupling method.