Role of Conductive Additives in the Electrochemical Performance of Composite Electrode for All-Solid-State Batteries

Abstract

All-solid-state batteries (ASSBs) with inorganic solid electrolytes (SEs) have recently attracted significant attention due to their potential application in electric vehicles. Particularly, sulfide-based SEs are regarded as promising candidate owing to their high ionic conductivities and good processability. In general, carbon-based conductive additives (CAs) are incorporated into solid composite electrodes to achieve rapid electron transfer and to maximize active-material (AM) utilization. However, CA creates distorted ionic pathways in solid composites unlike the case of conventional electrodes with liquid electrolytes, which may undermine the advantages of CAs under certain condition. Furthermore, CAs are known to accelerate the unwanted side-reaction of sulfide SEs with poor electrochemical stability at high voltages. Herein, we examine the roles of the one-dimensional CA (vapor-grown carbon fibers, VGCFs) on the electrochemical performance of composite electrodes with high AM fraction (fAM) (i.e., low SE fraction) based on impedance decoupling analyses. The CA provides a beneficial effect on the performance of the low-fAM electrode (fAM = 72 wt%) by enhancing its electronic conductivity, whereas the CA-incorporated high-fAM electrode (fAM = 88 wt%) shows accelerated performance degradation compared to the CA-free electrode. In-depth electrochemical analyses indicate that in high-fAM electrodes with high CA-to-SE ratios, the CA makes the ionic pathway considerably tortuous and accelerates the formation of SE-derived resistive phases, thus nullifying the advantages of CAs associated with improved electronic transport. In addition to the construction of optimized charge transport pathways, this study highlights the importance of the compatibility between the CA and SE, which is experimentally demonstrated by high-fAM electrodes with halide SEs.