Intrinsic charged defects and lithium-ion transport mechanisms in CdPS3

Recent studies have unveiled that the electronically insulating ${\mathrm{Cd}\mathrm{PS}}_{3}$ can serve as a versatile solid ion conductor, exhibiting exceptionally high conductivity at room temperature towards various cations. $\mathrm{Cd}$ 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 $\mathrm{Cd}$-vacancy and $\mathrm{Li}$-ion intercalation using first-principles calculations based on density-functional theory. We found that under a $\mathrm{Cd}\text{\ensuremath{-}}$ or $\mathrm{P}$-poor chemical potential, as the Fermi energy approaches the conduction band of ${\mathrm{Cd}\mathrm{PS}}_{3}$, the formation of $\mathrm{Cd}$ vacancies with the \ensuremath{-}1 charge state ${V}_{\text{Cd}}^{\ensuremath{-}}$ is energetically favored. The reason for achieving high electrical conductivity is the introduction of a high density of $\mathrm{Li}$ ions into $\mathrm{Cd}$ vacancies. Keeping the concentration (x) of $\mathrm{Cd}$ vacancies below 0.5 in ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{PS}}_{3}{\mathrm{Li}}_{2x}$ is crucial to ensure high $\mathrm{Li}$-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 $\mathrm{Li}$ ions. In particular, the $\mathrm{Li}$-ion diffusion barrier in ${\mathrm{Cd}}_{0.833}{\mathrm{PS}}_{3}{\mathrm{Li}}_{0.333}$ is determined to be 0.2 eV, which is highly consistent with the experimental value of 0.21 \ifmmode\pm\else\textpm\fi{} 0.022 eV. Further analysis revealed that $\mathrm{Li}$ ions on the surface show a preference for moving along the $\mathrm{Cd}$ or $\mathrm{S}$ atoms rather than the $\mathrm{P}$ 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.