Conflicting roles of conductive additives in controlling cathode performance in all-solid-state batteries

•Conflicting roles of conductive additives in all-solid-state batteries are studied.•Conductive additives are effective for enhancing electronic transport.•Conductive additives make the ionic transport and interfacial reaction sluggish.•Critical cell degradation factors are identified by impedance decoupling.•The compatibility between the conductive additive and solid electrolyte is crucial. Sulfide-based all-solid-state batteries (ASSBs) have recently attracted significant attention owing to the high ionic conductivities and mechanical ductilities of sulfide solid electrolytes (SEs). In general, carbon-based conductive additives (CAs) are incorporated into solid composite electrodes to enable facile electronic transport and to enhance active-material (AM) utilization. Herein, we reveal the conflicting roles of the one-dimensional (1D) CA (vapor-grown carbon fibers) in determining 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 reducing its electronic resistance, whereas the CA-incorporated high-fAM electrode (fAM = 88 wt%) shows inferior rate-capability and severe capacity decay compared to the CA-free electrode. In-depth impedance decoupling analyses indicate that in high-fAM electrodes with high CA-to-SE ratios, the CA makes the ionic pathway tortuous and accelerates the formation of SE-derived resistive phases, thus nullifying the benefits of enhanced 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-type SEs. [Display omitted]