Intrinsic charged defects and lithium-ion transport mechanisms in CdPS3

Recent studies have unveiled that the electronically insulating CdPS₃ can serve as a versatile solid ion conductor, exhibiting exceptionally high conductivity at room temperature towards various cations. Cd vacancies are believed to play a crucial role in achieving high ionic conductivity, but their specific assisting mechanisms remain unknown. Here, we investigated the synergistic mechanism of Cd-vacancy and Li-ion intercalation using first-principles calculations based on density-functional theory. We found that under a Cd- or P-poor chemical potential, as the Fermi energy approaches the conduction band of CdPS₃, the formation of Cd vacancies with the {-}1 charge state VCd^- is energetically favored. The reason for achieving high electrical conductivity is the introduction of a high density of Li ions into Cd vacancies. Keeping the concentration (x) of Cd vacancies below 0.5 in Cd_1-xPS₃Li₂ₓ is crucial to ensure high Li-ion conductivity, as concentrations above this threshold lead to a loss of long-range thermal stability and increased energy barriers that impede the diffusion of Li ions. In particular, the Li-ion diffusion barrier in Cd0.833PS₃Li0.333 is determined to be 0.2 eV, which is highly consistent with the experimental value of 0.21 ±{} 0.022 eV. Further analysis revealed that Li ions on the surface show a preference for moving along the Cd or S atoms rather than the P atoms. Our research provides insights into the role of vacancies in facilitating cation diffusion, establishing a theoretical framework for the design of high-ionic-conductivity materials.