Dual polyanion mechanism for superionic transport in BH${}_4$-based argyrodites

Polyanion rotations are often linked to cation diffusion, but the study of multiple polyanion systems is scarce due to the complexities in experimentally determining their dynamic interactions. This work focuses on BH${}_4$-based argyrodites, synthesized to achieve a high conductivity of 11 mS cm${}^{-1}$. Advanced tools, including high-resolution X-ray diffraction, neutron pair distribution function analysis, and mutinuclear magic-angle-spinning nuclear magnetic resonance (NMR) spectroscopy and relaxometry, along with theoretical calculations, are employed to unravel the dynamic intricacies among the dual polyanion lattice and active charge carriers. The findings reveal that the anion sublattice of Li${}_{5.07}$PS${}_{4.07}$(BH${}_4$)${}_{1.93}$ affords an even temporal distribution of Li among PS${}_4$${}^{3-}$ and BH${}_4$${}^{-}$, suggesting minimal trapping of the charge carriers. Moreover, the NMR relaxometry unveils rapid BH${}_4$${}^{-}$ rotation on the order of ∼GHz, affecting the slower rotation of neighboring PS${}_4$${}^{3-}$ at ∼100 MHz. The PS${}_4$${}^{3-}$ rotation synchronizes with Li${}^{+}$ motion and drives superionic transport. Thus, the PS${}_4$${}^{3-}$ and BH${}_4$${}^{-}$ polyanions act as two-staged dual motors, facilitating rapid Li${}^{+}$ diffusion. (© 2024 Wiley‐VCH GmbH)