• Publisher-DOI

Superionic conductors are key components of solid‐state batteries (SSBs). Multicomponent or high‐entropy materials, offering a vast compositional space for tailoring properties, have recently attracted attention as novel solid electrolytes (SEs). However, the influence of synthetic parameters on ionic conductivity in compositionally complex SEs has not yet been investigated. Herein, the effect of cooling rate after high‐temperature annealing on charge transport in the multicationic substituted lithium argyrodite Li₆.₅[P₀.₂₅Si₀.₂₅Ge₀.₂₅Sb₀.₂₅]S₅I is reported. It is demonstrated that a room‐temperature ionic conductivity of ∼12 mS cm⁻¹ can be achieved upon cooling at a moderate rate, superior to that of fast‐ and slow‐cooled samples. To rationalize the findings, the material is probed using powder diffraction, nuclear magnetic resonance and X‐ray photoelectron spectroscopy combined with electrochemical methods. In the case of moderate cooling rate, favorable structural (bulk) and compositional (surface) characteristics for lithium diffusion evolve. Li₆.₅[P₀.₂₅Si₀.₂₅Ge₀.₂₅Sb₀.₂₅]S₅I is also electrochemically tested in pellet‐type SSBs with a layered Ni‐rich oxide cathode. Although delivering larger specific capacities than Li₆PS₅Cl‐based cells at high current rates, the lower (electro)chemical stability of the high‐entropy Li‐ion conductor led to pronounced capacity fading. The research data indicate that subtle changes in bulk structure and surface composition strongly affect the electrical conductivity of high‐entropy lithium argyrodites.