The Impact of Cobalt Substitution on the Electrochemical Properties of Cubic Li 6.6 La 3 Zr 1.6 Ta 0.4 O 12 Solid Electrolyte for All‐Solid‐State Li‐Ion Batteries

Garnet‐type Li 7 La 3 Zr 2 O 12 (LLZO) is a promising solid electrolyte for all‐solid‐state lithium batteries (SSLB). The diffusion of cobalt (Co) into LLZO in combination with LiCoO 2 (LCO) as the active cathode material (CAM) is widely reported in FAST/SPS‐sintered LCO/LLZO composite cathodes, sintered under an argon (Ar) atmosphere, and associated with degradation. In contrast, air‐sintered LCO/LLZO composite cathodes often demonstrate greater cycle stability, indicating an impact of the processing atmosphere. To address this issue, this study systematically investigates the effect of Co substitution in Li 6.6 La 3 Zr 1.6 Ta 0.4 O 12 (LLZTO) sintered under both atmospheres. The structural, microstructural, and electrochemical properties of the sintered materials were characterized using XRD, SEM‐EDS, Raman spectroscopy, and EIS. DFT calculations were further performed to investigate the thermodynamic stability of Co substitution at different cation sites within the LLZO structure. Our results demonstrate that Co incorporation into the LLZTO lattice is significantly greater under Ar, with Co 2+ occupying Li‐positions according to DFT, and total conductivity is reduced. When sintered in air, most of the Co segregated as a secondary phase of LCO and electronic conductivity is increased. These results identify one possible contribution to the degradation of LCO/LLZTO composite cathodes processed under reducing conditions and suggest that post‐annealing in air may reverse it.