Charge transport kinetics in densified composite cathodes containing vapor-grown carbon fiber for sulfide-based solid-state batteries

Solid-state batteries (SSBs) utilize sulfide solid electrolytes (SSEs) instead of flammable liquid electrolytes used in conventional Li-ion batteries, providing potential benefits in terms of safety and energy density. However, despite their advantages, SSBs are impeded by several challenges, including the utilization of composite electrodes with interface instability between SSEs and other components, primarily due to carbon additives. In this study, a composite cathode comprising single-crystalline LiNi_0.8Co_0.15Al_0.05O₂, Li₆PS₅Cl, and vapor-grown carbon fiber (VGCF, 0 wt%‒3 wt%) is fabricated. Furthermore, the effect of carbon additives on microstructure evolution and interface stability is investigated to determine the relationship between charge transport characteristics and electrochemical properties. The results confirm that composite cathodes containing VGCF improve the initial discharge capacity owing to the high electronic conductivity of VGCF at low C-rates. However, at high C-rates, VGCF causes a “bottleneck” in the Li⁺ transport kinetics shortening the transport path. The results suggest the significance of appropriate dispersion of VGCF and densification of composite cathodes for achieving high power density in SSBs.